They think that they are successfully defying society's standards just because nobody actively criticises them. They think that the tolerance people have in the interests of not getting involved means they give consent. In fatty's mind, if nobody beats the crap out of you for having diabetes, immobility, and clogged arteries, that means they are silently worshipping such a brave, counter-culture protest. Really, people just don't want anything to do with someone so obviously unstable and stupid.