I think it is acceptable for friends to call each other names, because it builds our their relationship. Plus if they really friends they're not going to say anything that is going to hurt your feelings. I believe people do it for fun because its a quick way to get a laugh. The reason I believe this is because me and my friends do it all the time no matter where we are, and we just laugh at each other. But with all the friends I had I've never weakened a relationship with them by calling them names. Every time we talk about each other it always builds our relationship, because to us its a sign of respect.