Attraction + compatibility + long term material
The attraction is that initial mutual sparkle, which is there from the beginning. Then you get to know each other. The long term material is probably what you mean by "boyfriend material" - it's the extra stuff like honesty, moral, loyalty, openness. That you are part of his life, he doesn't appear to hide things from you, he doesn't have a large set of player friends, he doesn't have a range of exes he refers to as "psycho".