Such characters are fairly common amongst people I've played with as well.
I've often thought of how to curtail this character aspect - not so much the 'traumatic' (it usually isn't) background, but the intentional distancing from everything (even other PCs...often leading to situations where the character is only in the party because the DM forced them to be, and to stay there).
I've come to the conclusion that the 'positive reinforcement' mentioned above is really the only good way to fight against this archetype. What I'm planning to do for my own future games is to simply try and make 'the world' more interactive. Friends and family will try to keep in touch with characters, and will often be happy to help them out when they can (even if it's as simple as given them a place to crash). Allies and mentors will be available to provide advice or resources if sought after, and sometimes even if not. Miscellaneous characters will try to becomes friends and allies, be it in search of romance, or adventure themselves, or out of a percieved debt, etc. And all these character might indeed end up threatened by enemies or turned against the characters, but I hope that the effect of these interactions is such that it isn't the 'out of the blue' situation that often happens when DMs do use the important people in a PC's life. These other characters (and places, or things) won't simply be forgotten until they appear in danger or as an enemy, those situations will unfold before the players' eyes, and due to their interactions and what they are doing. As such, I hope that even if I do decide to use a brother as an [insert evil villain here], they will care about it because they've been actively involved with the character...or even caused it to happen in the first place, and they know it. And it won't be just a matter of yoinking the free, cliche plot hook.
That's the goal, anyway. I'm also hoping it will encourage more 'talkie' roleplay in general. It seems like a somewhat daunting task, but having these friends and family as active participants in the game, rather than mostly ignored names on paper, strikes me as a way to move away from the 'avoid society at all costs' archetype, while providing some interesting gameplay to present to the loner types, and everyone else.
So I guess I agree with Bendris, hong, etc

I hope that this little experiment bears fruit for me, and if it doesn't, provides something that helps you out.