I don't believe Steph is a selfish player. He has many attributes (like penetrating) and, on the other hand, many weaknesses (like defense). But he's definitely not a selfish player.
IMO, the problem with Steph in becoming a true PG is that he needs to be more effective with his teammates on plays other than pick and rolls, which is where he gets the majority of his assists. That's where he can really make his teammates better. I'm really hoping to see him be more effective on fast breaks, and if he concentrates on limiting the opposing PG's performance, then that will make his teammates much better.