He most likely has EXTREMELY high expectations of people, which is why he welcomes strangers, they have not had the chance to disappoint him in his own mind.

Once he's gotten to know somebody, he may (in his mind) of felt betrayed or something. Maybe he is offended easily, low self esteem?

I don't know the guy, but social hermits like making new friends - because they can get rid of them easy if they want to be alone.