It's a combination of factors IMHO.
- Cynicism exaggerated to its extreme limit. People like this have managed to convince themselves that everyone -- absolutely everyone -- has a major flaw, and the harder that flaw is to find, the worse it must be. The absence of an obvious flaw cannot be the result of a person simply not having any. It must instead be the result of meticulous conscious effort to hide it. The idea of a person being just generally "good" and not a total fuckup is alien to them, likely because their life choices have resulted in rarely (if ever) meeting any such people and being aware of it.
- Lack of self-confidence. These people have little to no genuine self-confidence (no matter how much they feign it publicly), and have real trouble accepting (or even considering) the possibility that maybe they really do deserve someone's kindness and affections without an ulterior motive.
- Trust issues. Tying into both cynicism and lack of self-confidence, these people are unable to truly trust anyone. They're convinced that anyone who expresses interest in them must have ulterior motives, and bizarrely the closer to "genuinely good person" someone seems to be, the less trustworthy they'll feel to people like this. They "must be after something." So people like this never let anyone get close enough to them to have a meaningful relationship.
