Lack of social skills
Highly sensitive temperament
Some people’s temperament is just more sensitive than others. They are easily hurt and confused when someone is thoughtless or is unable to give them enough time or attention. They take too many things far too personally which makes them very vulnerable causing them to stay where they feel safe.
Some people just never learned how to initiate contact with new people or maintain friendships. Maybe they grew up in families that were isolated for one reason or another, but the result is an adult who feels awkward around others and who doesn’t understand the give and take of social relationships.