It is easy to become attached to someone. Either you instantly click or you spend some time together and before you know it, you get to know and understand the goals, ideals, hopes and dreams of the other person (at least to some extent). They stop being a stranger, a part of the faceless mass, and start being a part of your world.
And yet, what matters the most and what defines everything is your motivation for getting close with someone and their motivation for getting to know you.