It seems unusual that Hashem picked such a beautiful sign to indicate to us that the world deserves to be destroyed. People are generally happy when they see a rainbow; it is used in stickers and all sorts of cute decorations. So why is the sign of bad news such a beautiful thing?

Rav David Zvi Hoffman explains that the rainbow is obviously not a sign for Hashem, because Hashem does not need a reminder not to destroy the world. Rather, it is a sign for people. If so, there must be something about a rainbow that makes it an appropriate symbol for the people viewing it.

Rav Meir Simcha of Lublin has an interesting comment on the Midrash that teaches why Hashem commanded Noach to build an ark. It took Noach 120 years to build the gigantic ark, the idea being that during this entire time, people would ask him what he was doing, which would give Noach the opportunity to warn them of the coming flood and convince them to do teshuvah. Rav Meir Simcha asks, how is it that Noach was not able to get anybody to repent for 120 years? Imagine a "kiruv professional" who does not have any success at all for more than a century! The answer he gives is that deep down, Noach felt that the people of his generation were horrible, hopeless sinners who were beyond redemption. His heart was not in it, so he could not convince anyone to repent.

Rav Soloveitchik explains that this is why Hashem chose a rainbow as a sign. Hashem was teaching Noach that he saw only the dark clouds, the sins of the people, but not the small amount of light that can interact with raindrops to make a beautiful rainbow. The generation of the flood could have been reached; they did have the ability to change their ways. People have the potential for great good and beauty, as symbolized by a rainbow. In a righteous generation, we do not need to be reminded of this; it is self evident. In a less righteous generation, however, we need to be reminded of the beauty hidden in every Jew, thereby encouraging us to reach out and help them, and ourselves, realize this great potential.