The female belly has a long, rich history as the focus of so many masterpieces. There are also physiological differences that inspire curiosity like the "mystery" of a pregnant belly.
In recent years, it has been increasingly popular for women to show off the belly they've been working in the gym so hard to flatten. Some even decorate their belly with artwork and jewelry.
With the amount of skin showing considered socially appropriate increasing with each generation, the natural response is an attraction to that new feature of the female form. Society begins to focus on it as demonstrated brilliantly in a recent television commercial depicting singing bellybuttons.
What attracts men really gets interesting when you start to study other socities. Everything from lip discs and neck rings to all out genital mutilation attracts those that grew up being expected to like it.
Finally, every time you like looking at something, you have an increased probability of liking it again. It's called positive reinforcement. It's the same principle that leads to kinks and fetishes.

