Trying too hard to be liked as an adolescent is a major factor. It makes someone a natural target for bullying, but it's worse because they have a hand in their own humiliation. At some point, the lonely individual may grow a spine, but by then it might be too late, adolescence is over and all those crucial experiences for healthy adjustment can't happen. Now they're stranded in adulthood with less experience than a teen.
A rough timeline could go something like:
Born with some Innate introversion, perhaps autism.
Overbearing/emasculating mother figure (absolutely terrible for a boy), abuse, or helicopter parenting.
Emotionally unstable/needy/insecure behaviour starts to get noticed by other kids by around middle school.
From there on bullying and social exclusion turns life into a daily torment.
The individual leaves school then wastes the remainder of their youth hiding away with gaming or drugs/alcohol to recover.
This goes on for years, eventually leading a person to grow out-of-sync with their age group. Their peer groups lifestyles and sensibilities change, but they do not, ensuring social isolation as an adult and of course total inability to function romantically with someone age appropriate.
