Why am I not happy with this girl?

After such a long time, I finally have my first girlfriend.But I don't know why I'm not as happy as I think I should be. In my opinion I believe it's human nature. She's sweet as hell, and perfect in soo many ways, but she isn't the best looker. And I really believe that if she was like some chicks I liked in the past who were "hotter" I'd be happier. God damn, I hate human nature. What can I do to resolve this? I really like this girl.