GUYS: why would you constantly insult a girl that's a good friend?

K, long story short...I'll be in the middle of texting my guy friend when he starts insulting me randomly (e.g. loser) and it will turn into a name calling "fight." I know he's just kidding, but he went so far as to call me a vulgar name once. O.o is this flirting? why is he doing this?