From a "partly BS - partly true" link that I ran into today...

Perhaps the biggest difference between the male and female brain is that men have a sexual pursuit area that is 2.5 times larger than the one in the female brain. Not only that, but beginning in their teens, they produce 200 to 250 percent more testosterone than they did during pre-adolescence. If testosterone were beer, a 9-year-old boy would be getting the equivalent of a cup a day. But a 15-year-old would be getting the equivalent of nearly two gallons a day. This fuels their sexual engines and makes it impossible for them to stop thinking about female body parts and sex.