Because I'm blunt and not particularly friendly until someone earns my respect. I also am not afraid to give idiots a verbal beating, and considering how many stupid people are in the world, this doesn't earn me a whole lot of fans. I've got a sarcastic, dry sense of humor and sometimes people think I'm being serious when it's tongue in cheek (this happens most often online because typed words don't convey tone very well). I have an INTP personality, so I realize I come across as arrogant or detached quite often when I'm simply off in my own thoughts and not paying attention.
But the friends I do have I'm very loyal and generous to.