Anyone else's toddler do this?

My 2 year old doesn't like to really interact with kids his age or a little older he only plays with adults?! How do I break him from this or should I just let him chose who he wants to play with? I mean it's not a HUGE concern but it is odd. He lets his little cousin pick and punch him and of course we fuss at her but if a kid takes his toy or something he lets them have it he doesn't fight back or anything but as for adults if I were to take a toy or something he would fight back and say MINE! Anyone else have this problem??