I think breed is part of it. Orpingtons are generally very people-friendly. Our Golden Comet are very friendly as well.
Some of it is individual personality, some of it's environment, some of it is what *you* do.
We started right off with hand feeding. The chicks were frightened of this huge strange hand showing up in their little box, but if we held very still, soon they realized that the hand brought food.
I've noticed that if we don't have time to spend picking up birds, hand feeding treats, etc, they get a little bit less trusting and need to be won back ... so I would say that consistency helps a lot as well.
Many of ours will come running when called, because they have come to expect peanuts. They'll mug guests from behind if they can see into a pocket while someone is sitting in a lawn chair ... might be a peanut in there!
My 7 year old girl has a golden comet she calls her "stunt chicken." She can carry it to the top of her playground slide and it will ride down in her lap, and launch itself into the air at the bottom. I thought she was tormenting the poor thing, but it will run around to the back of the slide with her and wait to be picked up. Craziest thing I ever saw from a critter! She routinely begs to let her favorite sleep in her bed with her. So far, that's not been allowed, but I keep expecting a smuggling operation one of these evenings.