I don't understand sarcasm. I mean, I know what it is and everything, and I can participate in a sarcastic exchange if I'm expecting it, but if I'm not paying attention, a remark meant to be sarcastic gets automatically processed as serious, and I respond defensively. Various people over the years have noticed this and used it to their own amusement. (Which I don't really mind. I was ignored enough as a kid that even just being teased is a sign of social inclusion for me and makes me feel warm and fuzzy.)