My one data point; the one ex-friend I had that was really encouraging/interested in things I talked about (at first) turned out to be a borderline sociopath and I had to end the friendship.
I've had similar experiences. I find some people use attention and the doling out of validation to establish control and eventually manipulation. They don't have to be sociopaths, even average narcissists often play such games.
It's very counter intuitive to question someone's motives when their external presentation is so disarming. Undoubtedly this keeps you from some genuine relationships as well though.
It is a gamble. The question typically boils down to whether you are willing to gamble. I mean.. it is genuinely harder to make 'real' friends as you grow older. As a kid, there is no question of ulterior motives, because they are in the open. He has a console and a new game I want to play. We are best friends now.
