Have you been diagnosed with depression? (Sorry if I'm already supposed to know this--I've not been using LJ much at all until a couple of days ago.) It sounds likely to me that you are. In which case, the answer is: brain chemistry. You might have already known that, and it's even likely that you were being rhetorical in the first place; I'm worse at picking up social cues from text even more than in face to face interactions.

Shall I shut up now?