A. You friended me out of the blue and yet don't seem very interested in me (i.e. we've never talked), or
B. I friended you but you never friended me back-- because I don't want to be a creepy stalker person at you and you should get to decide who you want friending you.
...However, I do tend to be spectacularly inept, so if you think it's weird that I unfriended you, tell me because it was probably me clicking the wrong thing again. D: