I'd guess self-esteem issues, combined with supportive internet fetish forums.
You actually have to admire it in a way. This guy has managed to create a situation where he never has to worry about a job or a relationship.
He'll never get hired, but the reason for him never getting hired is his fetish.
He'll never be in a decent relationship, but the reason for him never getting into a decent relationship is his fetish.
He's basically managed to eliminate two massive stressors from his life. Unless he's working at a specialized store that caters to people with his exact, obscure fetish then even trying to get a job is pointless. Unless he's forming a relationship with someone who also gets off on going out in public wearing nothing but a shirt and diaper, and acting like a toddler, even trying to talk to a girl is pointless.
In his fucked-up mind, people don't hate him because he's an asshole, he can't get a woman because he's a weirdo, and he doesn't get fired because he's a shitty worker; all of those things are because people are intolerant of his fetish.
The internet has had a terrible effect on the mentally ill.