At the end of each day of Creation, HaShem looked at what He had done so far and "saw that it was good." The Creation was good - there was light and darkness, heaven and earth, plants and animals - but it wasn't yet complete. It was only after HaShem created Adam and Chavah that the Torah tells us Vayechulu hashamayim veharetz, "And the heavens and the earth were completed."

Though the heavens and the earth were created on the second and third days, they - and indeed the entire creation - were not considered complete until Adam and Chavah were created.

Why is this so?

After all, Adam and Chavah were only two people. That seems so small when compared to all the animals, plants, water and other things in the universe. Yet when HaShem created man, it is as if He said: "This is what I had in mind when I started creating the world." Not because there were going to be more people than anything else, but because people could be more special.

What makes people so special?

People have seichel; they can think and understand.

But wait a minute. Animals can understand things too. They can figure out how to get food, how to keep warm and how to take care of themselves. Some can even be taught skills and tricks.

But animals can only use their seichel to do things that they want for themselves. Animals can't think about ideas that are higher than themselves. People can. And as a matter of fact - people should.

When HaShem created Adam and Chavah, He wanted them to use their seichel, not just like animals, but to realize that there are things which are higher than seichel, things which we know are there, but don't understand because they are holy and spiritual.

That's why HaShem was so pleased when Adam declared to the world: "Let us bow down to HaShem." Adam had used his seichel to realize that there is a Creator, and that everything should serve Him. When Adam said this, HaShem proclaimed the world completed. This understanding is what He wanted from the first man, and this is what He has wanted from every human being ever since.

(Adapted from Likkutei Sichos, Vol. IX, pg. 454ff)