Think of the Hulk as a four yer old. When a four year old is having tantrum, the worst thing you can do is to tell them to clam down Right Now. That just makes the tantrum worse.
When he first turns into the Hulk on the Helicarrier, notice how it takes a long time to change. I think it was because he was trying to force himself to calm down. That makes The Other Guy want to come out more. And then you have Black Widow telling him that he calm down. He Knows That. The Other Guy is getting pissed at her, too. He finally yells at her to get away, and he loses control. The Hulk comes out, and where’s that puny human who wouldn’t shut up?
At the end, Banner doesn’t control the Hulk so much as aim him. He wills the change, which comes very quickly and smoothly.
My $0.02.