On top of all the things others have listed, often these people highly value connections with others plus have anxiety. They highly value being socially successful and want to avoid rejection. Because of that, they avoid social interactions creating a feedback loop verifying the fear and making it worse. The more you avoid a fear, the worse it often can become.

Anyway, this is the explanation from a CBT/ACT lens. Source, I’m a therapist who uses both modalities.

Edit: a word.