At the time, I had a moral system in which it was not permissible to leave grad school because science was the thing I should be doing. However, towards the end of my first year I became too depressed to do any problem sets and as a result I had to drop all my classes at the last minute and would then have had to reapply to get back in, which wasn't happening. If I'd been slightly less vulnerable to stress-related depression, I suppose I'd still be there (and still be quite unhappy, so maybe the whole thing was adaptive after all).

I don't have a good link to post, but if I write more extensively I'll put it here.