A lot of times, of course, it really isn't so much a long story. This gets said for several reasons (apart from story length):
1) I don't feel like going into it. Dammit, not this mutha . . . I don't feel like dealing with you right now. Go away
2) The incident makes me look stupid, and I can't make it sound funny enough to warrant telling you about it.
3) It's really not that interesting, but I want to sound mysterious because chicks dig dangerous guys. So, rather than reveal the fact that I am boring, and possibly a loser, or lying about it (takes too much effort and too good a memory), I just act secretive and let your imagination fill in the blanks. (Of course, she's really probably thinking what a loser anyway, but hope springs eternal.