Isolation. I don't do too well with too much solitude. My second apartment in college, my parents insisted I live in a studio apartment on my own, apparently because they had taken it into their head that this would result in paying LESS for rent (because...don't even ask me, sometimes my parents are just legit dumb). This was a colossal mistake, the only time I have taken worse care of myself than during the time between moving in and getting a couple of rats was this one time when I was just straight up too sick to care for myself properly, which coincidentally was not long before moving out of my first apartment.
Ironically, I get really nervy when I'm around too many people I don't know, so I tend to avoid big groups if I don't have friends as backup. Sort of a self-perpetuating problem, as it were.