Why are men more aggressive then women?

Because men want to be the leader. Men are known to be tough, so if they were like girls that would ruin their attractiveness. It also has to do with different levels of testosterone.

Whether men are actually more aggressive or not depends on the context. If you threaten a woman's children or get between her and her man and you will find plenty of female aggression. While there are more men in prison, many women who are in women's prisons tend to be quite aggressive.