Why do some couples insult each other for fun?

My best friend and her boyfriend jokingly insult each other and take it very well. She called him a fucking bastard and he called her a bitch for fun for the fun of it. I don't see how that is fun at all. My ex boyfriend did that to me and I wasn't happy. He said he was going to break up with me because of my size and I was just crying my eyes out. Then he said, "Girl I was just messing with you. That shows you right there you don't know your own boyfriend. You better stop taking it personally! I joke it's what I do. I cried so hard he even said, "Whatever crybaby. I like a woman that can take criticism and have a great sense of humor since you don't."