The Holiday Blues?
I had a weird bout of depression earlier today while out with friends. I felt better as the day went on but weirdly it cleared up a bit more now that I'm alone.
It wasn't my friends depressing me or saying hurtful things. I think I just needed a general break from socializing. I'd been busy with people and hadn't had a chunk of alone time in almost a week. Combine that lack of alone time with some odd relationship feelings lately and I just wanted to chill at home and get my head straight.
Sometimes that's really what you need, I think. It's also one of the reasons riding the bus (or train) can be so nice.