I remember why. Because when I do smile, or generally look like I'm not sad, serious, or having a bad day... people talk to me. All. The. Time.
When I look to be in a slightly-right-of-neutral to pleasant mood, people approach me constantly to chat and I can't get anything done.
I learned this a long time ago. Especially at the gym. If I'm at the gym and don't keep my eyes straight ahead with no expression (no eye contact, no small smiles at people that happen to pass by) and generally don't look preoccupied; people do not leave me alone.
It's like when you're reading and people automatically assume that you're "doing nothing". Just because I'm reading a book does not translate to: I'm bored, please entertain me. Just like "I'm breathing and not looking angry" doesn't mean: Flex your muscles and try to flirt with me. Grrr.
I think I need to put more effort into an expression that says: Blank, but preoccupied.