Personally, I find being on top very pleasurable, but I'm sure it varies from woman to woman.
I think the main reasons some girls don't like being on top are:
1) You feel more exposed, which can be a problem if the girl is shy about her body.
2) Not all girls are comfortable being "in control".
3) It's more work.
Well I don't know about other girls but I really enjoy being on top. It let's me control the speed and depth that we do. I also really like being in control and every guy I have been with loves it too so it works great for me and my guy. Doggy style is my favorite position but cowgirl definitely comes in second place for me:)