Social anxiety is caused by a failure to experience strong social rejection by individual peers, e.g. heartbreak, and is exacerbated by experiencing moderate social rejection by groups of peers.
Normal children form bonds with specific individuals that are significantly stronger than bonds to other friends - best friends and boy/girlfriends. These relationships inevitability end causing emotional trauma beyond what's possible from less intense relationships. These heartbreak incidents desensitize the individual to social rejection from normal people and groups.
Some children don't form partner-bonds and so never experience that strength of social rejection. Often that's combined with difficultly forming friendships and joining peer groups, which causes unusually strong social rejection from normal people. This results in associating the worst emotional pain with interactions with normal people.
People who've experienced heartbreak know (are conditioned) that normal social settings aren't able to cause the worst hurt and so aren't anxious in them. People who haven't experienced heartbreak think (are conditioned) that the worst pain is possible from normal social settings and so can become anxoius in them.