The failure of fathers. Men don't understand how to be men and thus can't teach their sons. The rise of feminism and the portrayal of weak, bumbling men in the media don't help, but IMHO it all boils down to lack of good fathers.
EDIT: Oh, and as human ills said in the post above, people do what they want to do. But that's a different discussion than why the modern American man is weak.
If I could be anywhere right now, I'd still be out there, on my bike, in the wilderness.
2010 Concours 14