1. Not everything in us need serve a purpose, even with evolution held as part of the picture. That said, novelty (like a book that changes your views of the world, or a beautiful painting) is necessary for us to learn and be able to adapt to our surroundings. On a less deep note, the ability to write good poetry, smear paint in an interesting way, or just plain enjoy it are methods for meeting other people, and therefore raise your chances to get laid.
  2. Novelty and personal bias.
  3. Horniness works if all you want to do is pop the little suckers out, but a couple that stays together gives a better chance of survival to their child. Also, romantic love is a part of mate selection, and so is likely to be passed on.
  4. Mate selection all the way. The more visible you are, the more members of the opposite sex see you, the more chances you have.
  5. Overpopulation control.

Largely bullshit? You bet. But given that evolution suggests that progress happens by accident, my guesses are as good as any you can make up and post with no form of background research whatsoever.

Dialogue: Thanks, and I wasn't suggesting you were. As my current educational task is getting out of high school and I haven't taken any biology for three years, I felt that the disclaimer was necessary.