Men can be fashionable too, it's just typically not as fun for us. I would say most of the time the only reason men are interested in fashion is to become more attractive to women.

Edit: I realize this kind of generalization probably offends people. I'm sorry, I just believe that men and women are fundamentally different from each other. Here's my google search findings on the topic:

http://www.lloydianaspects.co.uk/evolve/fashion.html