Why is my cockatiel always angry at me?

My cockatiel is three months old, and hasn't quite figured flying out yet. He was hand-raised and spent most of his time with people in the store I got him from, so my tiel is used to people, and adapted quickly. He doesn't fear my hand, allows me to pick him up when he wants to go somewhere. I spend tons of time with him, and he trusts me, but still doesn't seem to like me. Whenever my hand approaches him, unless he wants to go somewhere or is frightened, his crest goes down and he begins to scream, which, of course, means anger. I pet him anyways, and he doesn't move or bite, but continues to scream at me, bobbing his head. Sometimes he does it even if I'm only looking at him. Why is he angry?