The way I see that sort of thing is that it's like wanting a certain kind of car and then seeing it everywhere. It's not that thinking about it is making the car pop up everywhere, but that you're attention is honed in on it. The same principle would be at work in visualizing the kind of person you want in your life. It's a little more challenging because people aren't cars and you can't just mentally fixate on a yellow VW bug person. But it does get you on the wavelength of the type of person you want to be around, and that way you're better able to spot them, or spot those qualities. It's like the brain is a marketing tool, similar to facebook, it will find matches for what it thinks you like based on your recent viewing history.