Because boys will ALWAYS be boys.... silly, annoying, but totally cute :)
He probably likes to see you get mad, because he thinks its cute, or just plain old simple: he likes you and wants to see your reaction on other guys.
Guys pretty much do that because they want to seem cool around you or something. Or because they don't want to lead on that they like you, so they want to act normal, or they just plainly want to sort out who you like.
guys like to compete and talk bad things about other guys to get higher status become a alpha male or to be a jerk or to gain a girls attention to him rather then the other guys trust me i know
Because if the guy teases you they like you and are jelous
Because they most likely have acrush on you themselves.