Wisdom. I don't know much about friendliness, as some are more curious and comfortable being at my feet and almost being stepped on. What I do like in a bird is Oregon Blues word, confidence. Add to that a certain regal calmness or business like demeanor and I love that bird. Yes, I breed for it. If I'm going to have birds around to keep and enjoy, I'd rather they be birds I like. The list of temperament traits I dislike is long. Birds with those traits don't make the breeding pens.In my opinion, temperament is extremely inheritable. However, I don't think you select for "friendliness" as much as you select for confidence and you remove timid, aggressive, or high strung birds from your breeding pool.
How friendly they are depends upon how they are raised and handled.