Why are ex boyfriends so mean?

my ex boyfriend and i have been broken up for a month now. i broke up with him, but he has been saying horrible things about me. he is just very immature about this break up and is acting super rude. i ran into him at the store the other day and he just looked at me gave me this sad look and turned around. i don't understand why he has to be so hasty when i am nice to him? i'd still like to remain his friend, he was a big part of my life.