A number of things. I had run my department by myself since November. A month ago I sat them down and asked for a raise and the title of department supervisor as I'd earned it (I had to completely rebuild the department from scratch, brought the lab up to OSHA regulations, cut turn around times in half, etc.) and they laughed me out of their office saying I'd lacked the experience they wanted. A day later they came back and said they would give me a formal review to "meet me in the middle." They then hired someone for the position of manager, and to this day havent told anyone he is the department manager (two weeks after working with him we discovered it by his email signature). The courtesy of telling me they hired someone to fill the position I felt I'd earned would have been nice. It's been a month and they haven't given me my formal review. I never received my yearly raise and since I sat down with them a month ago they've been harassing me every single day I work with them.

All good and fine, I know none of what they're doing is illegal and I know they don't owe me anything. Friday was a particularly bad day (as far as insulting me; every half hour he insulted my intelligence while simultaneously giving me more work) and on his way out the door the owner couldn't even leave the building without a little mud-slinging. So I snapped and put my notice in.

Tl;dr: I valued my self worth more than my rationality in a moment of frustration. Not my smartest move but also the one I regret the least.