Inspired by another thread (thread about a thread, sorry) which made me think maybe that's why I don't make friends with all and sundry but rather keep myself to myself and stick to the friends I know well.
I moved around a lot as a child but even when we were settled in one place for longer periods of time I didn't really hang on to friends. I have 2 friends from secondary school who I see infrequently and no friends from any of my primary school.
My parents were never really involved with other parents and I didn't have other kids come to the house to play with me.