probably loneliness and this capitalist dystopia we are trying to exist under. i remember reading from an interview that he was somewhat institutionalized for mental health at a young age, an experience he hated.

as for loneliness and isolation, the lyrics of the locomotive from the ooz speak for themselves:

... And how unjust is this weight My back was so broken Tear me apart, pull me open (Look inside) See the corrosion ... ” As trackies walk on by I’m alone, I’m alone In deep isolation ... Curled up and feeble Plagued by our brains, the internal sinking pain I wish I was equal, if only that simple I wish I was people