Rosh Chodesh Sivan marks the day when the Bnei Yisrael came to the Sinai desert, midbar Sinai. This location was chosen by G-d as the fitting place for the Torah to be given, and therefore, immediately upon arriving, Moshe began preparing them to receive the Torah. This same connection between the place (midbar Sinai) and the event also finds expression in the fact that Parshas Bamidbar is always read before Shavuos.
There are two reasons given to explain why Parshas Bamidbar is always read before Shavuos. First of all, a midbar (desert) has no owner. Even a public domain has owners -- it's just that everyone owns it equally. In contrast, a desert has no owner whatsoever. The Torah was therefore given in a desert, to teach us that whoever wishes to receive the Torah is free to do so.
A second reason is connected with the curses of Parshas Bechukosai. The Gemara (Megillah 31b) says that since Shavuos is considered to be like the beginning of the year, Ezra established that these curses be read before Shavuos, "to finish the year and its curses." According to this reason, Bamidbar is read before Shavuos to provide a respite between the curses and Shavuos.
Both of these reasons need explanation. According to the first reason, the Torah was given in a midbar to stress that the Torah is ownerless and that everyone has equal access to it. However, the Torah was given exclusively to the Jewish people! It would have seemed more fitting for it to be given in a private place, or at least a public place which was in the communal possession of all Jews.
In the second reason, the main connection is between Shavuos and Parshas (Behar) Bechukosai, and the placement of Parshas Bamidbar is only incidental. Nevertheless, Parshas Behar-Bechukosai begins speaking about Mount Sinai (har Sinai) rather than midbar Sinai. Since it is relevant to know that the Torah was given in a desert, why doesn't Bechukosai begin with mention of midbar Sinai instead of Mount Sinai?
This can be explained by first analyzing the statement in the beginning of the fifth chapter of Pirkei Avos (which we read this Shabbos), "The world was created by means of ten Divine utterances. What does this come to teach us, for indeed, it could have been created by one utterance? But it was so to bring retribution upon the wicked who destroy the world which was created by ten utterances, and to bestow ample reward upon the righteous who sustain the world which was created by ten utterances."
The well-known question on this Mishnah is that if the world could have been created with one utterance, it is therefore only "worth," so to speak, one utterance. Why do the wicked deserve more punishment (and the righteous more reward) if the world is in reality only "worth" one utterance?
The explanation is that there are two dimensions to the way in which G-d created the world: one which is called "one utterance" and the other called "ten utterances." Before there was differentiation between the various types of creations, G-d created an unformed existence (metzius yesh). Only later was this yesh formed into the universe as we now know it. The first act of creating the yesh was done through the "one utterance." The creative process which brought about all the particular types of creations is referred to as the "ten utterances."
One difference between these two dimensions is that through the "ten utterances," importance was ascribed to all of the individual creations. The emphasis was on the creation itself. On the level of the "one utterance," however, the creation is of relatively little importance. The main revelation is that of G-dliness.
These same two levels of revelation are reflected in the Torah, as expressed in the verse (Psalms 62:12), "G-d spoke one, I heard two." The "one" refers to the dimension of Torah which is united with G-d and higher than the world, similar to the level of the "one utterance." The "two" refers to the level of Torah which deals with worldly matters, similar to the "ten utterances." In practical terms, the first level corresponds to the blessing we make over the Torah, which stresses how the Torah is connected to G-d. The second level which is connected with the world finds expression in the laws of the Torah, which deals with worldly matters.
We can express this in more general terms. The first level (of the creation and of Torah) represents a revelation of G-dliness and the consequent nullification of the universe. This is the idea of a hala'ah (elevation) "from below to Above." The second level represents the hamshachah (drawing down) "from Above to below" to penetrate the universe with G-dliness.
Each of these two types of revelations has an advantage over the other. Through the "ten utterances," which is the drawing down of G-dliness, the universe becomes imbued with G-dliness. However it is only a low level of G-dliness, one which the universe is able to withstand. The revelation of the "one utterance" is much higher, but -- for this reason -- it doesn't affect the universe.
These two types of revelation are alluded to in the two Torah portions Behar and Bechukosai, and in particular to their names. Among the various types of inorganic matter, a mountain (har) is in a way similar to organic matter (since more earth falls upon it and it "grows," so to speak). This represents the growth and adaptation characteristic of the universe, "ten utterances." Bechukosai, on the other hand, comes from the word "engraved" (chakikah). Unlike letters which are written with ink on paper, engraved letters have no existence independent of the rock in which they are engraved. This nullification of the letters corresponds to the nullification of the universe which corresponds to the revelation of the "one utterance."
The ultimate revelation is when both these advantages are present. This is the idea of the expression dirah b'tachtonim ("dwelling place in the lower worlds"). The word dirah indicates a revelation of the essence of G-d, whereas tachtonim emphasizes the lower worlds. Having both together indicates that this highest revelation has the ability to penetrate the lower worlds.
The way to combine both these G-dly revelations is through a revelation which is higher than them both. This third revelation corresponds to the midbar (desert). The reason for this is because a desert is unfit for human habitation. This can be taken in a negative way, i.e. because it is so low that it is not suitable for humans. It can also be seen in a positive light, i.e. that it transcends anything a human being could possibly reach.
According to this, we can answer both of the questions which we originally asked. Parshas Behar does not begin with the mention of the desert because it expresses the "ten utterances" which are on the level of the world. On the contrary; it stresses the significance of worldly existence, and therefore mentions a mountain (Mount Sinai) which is the opposite of nullifying existence. Parshas Bechukosai emphasizes the other extreme, the nullification of existence, as mentioned above. After dealing with both extremes, the Torah then has Parshas Bamidbar, which is higher than these two extremes and therefore has the ability to combine them both.
This also explains why the Torah was given in a desert rather than in a place owned by Jews. A place of communal ownership corresponds to the level of Torah which is within the grasp of the Jewish people ("ten utterances"). The Torah was given in a desert in order to allude to the higher dimension of Torah which is completely beyond human grasp. In this way we receive the dimension of Torah which is completely united with G-d. This will be accomplished completely in the days of Mashiach, when (G-d said), "A new Torah will come out from Me" (Isaiah 51:4). The word iti ("from Me") refers to the Torah as it is completely united with G-d. This level will nevertheless "come out" to the level of each individual.
The preparation for the giving of the Torah was the unification of the Jewish people in the Sinai desert. This is alluded to in the verse vayichan sham Yisrael ("and the Jews encamped there"), where the word vayichan is in singular tense ("and he encamped"). This indicates that the Jews became united a single person. The way to attain this level of unity and ahavas Yisrael is through self-nullification. Only through bittul is it possible to avoid strife and achieve true ahavas Yisrael.
On the other hand, we see that receiving the Torah is also connected to a certain degree with the person feeling his own existence. He must learn Torah with a full measure of understanding, bringing the Torah to the level of his own intellect. We therefore see again the two extremes similar to the "ten utterances" (connected with a feeling of the importance of individual existence), and the "one utterance" (connected with the bittul of all existence).
The same two extremes are found in Parshas Bamidbar itself. On the one hand, a midbar represents the nullification of existence, as explained above. On the other hand, Bamidbar contains the counting of the Jewish people, which stresses the importance of the existence of each individual.
These extremes also correspond to the two reasons for reading Parshas Bamidbar before Shavuos. The first reason -- in order for it to be given in an ownerless location -- corresponds to the idea of bittul, as explained above. The second reason -- in order to intervene between the curses and the giving of the Torah -- corresponds to the importance of individual existence. This is because G-d wants us to have the full measure of blessings, primarily the blessings which enable us to fill the world with G-dliness and bring the redemption. This stresses the importance of the G-dly service of each individual.
This is also connected with the two practical directives which come out of this gathering. First of all, this is an auspicious time to gather more and more people together on Shabbos. If this is a vital activity every Shabbos, how much moreso on the Shabbos which blesses the month which contains Shavuos! Secondly, it should be publicized everywhere possible the necessity of bringing all Jewish children, even the smallest babies, to the reading of the Ten Commandments on Shavuos.
Here again we find the expression of these two extremes. The gathering together of Jews on Shabbos, and the inclusion of even the smallest children in the reading of the Ten Commandments emphasizes the unity and nullification of all Jews. At the same time it stresses the other extreme, since every Jew is so important that every individual must be included, and must receive the Torah on his or her own level.
May it be G-d's will that our good resolutions to increase in Jewish unity bring about the immediate redemption, so that we can celebrate Shavuos in the most complete manner -- in the Beis HaMikdash in Jerusalem, immediately.