Some things are just meant to be... but it is still up to the person/s involved or are under those circumstances, how and what angle they will tackle the opportunities presented. I think this is very right, opportunities will present themselves and it is up to the person/s involved to decide how they will handle it. I put a lot of emphasis on the way the camera also comes into play in certain scenes - and particularly under the street lamp, you see the exposure blown out for a second and a closer zoom-in on Brian's face as we watch Justin walk out of the shadows. For me that's the moment where the opportunity presents itself and Brian has to decide what to do with it. Clearly he makes the right choice going over ;) This kid that was worth changing his plans for the evening happens to also change him.
But I think at the very beginning, Brian might have seen or felt something, that beyond the innocence and the fragile looking teenager is something worth unraveling, be it courage, sweet innocence, just something Yes, I think this is where I stand on this question after thinking it over and over. I've spoken to people who have been together for 25 years who told me that there was some instant connection the moment they met them. It wasn't quite a romantic attraction (It's totally the -strictly friends for years, dating other people along the way, and then feelings developed- story); but the way they described it, it just felt like they realized there was something about this person from the moment they were introduced, that they were special in some way. I think that's true for just certain people you meet in your life in general - friends and lovers. So while I'm sure Brian didn't take this kid home going "Oh, he'll be my one true love," he (at least in my romantic mind) felt that there was something special about him, something he had to have in that moment.