What words are 'offensive' or 'friendly' or whatever are defined by our social group. For teens, in particular, they are often trying to develop some kind of 'in group' identity or process of identification that distinguishes themselves (most notably) from older generations and from their parents. Taking something that their parents find offensive and 'reclaiming' it as a term of 'affection' for each other is one thing. Though... I do agree that often people don't seem to speak very fondly to their friends... I guess I think that that is a different issue from language, though.