Because society has taught them (stupidly) that being mean is considered manly.
It is not.
It's always the woman's fault. Have you learned nothing from Man school?
They miss their mummy.
Ohhhhh! That is soooooo misandrist. (Hint: Misandry is mean.)
Treat em mean to keep em keen?
Probably frustrated because they have too much testosterone in their balls..
You left the nice guys back in the friend zone remember? (:
Because they are not sure of their own masculinity, and they are too stupid to find any other way to prove how manly they are.
They are weak, and cowardly.