Insecurity

Tony Stark has many inner demons - too many to count. One of those demons was the privatized insecurity of serious relationships that he hid behind playboy charm and a sarcastic demeanor. Because he was Tony Stark. He wasn't allowed to be insecure about relationships. But he was. Every serious relationship he had ever attempted had ended bitterly all because of him - stupid, idiot, insecure Tony. He didn't want Bruce to be a part of those attempts. But Bruce was determined. And Tony couldn't fight him.

Headcanon: Bruce knows that Tony doesn't trust his feelings with just anyone, because his heart has been broken too many times in the past few years to make him bother. But he knows damn well when his friend needs someone to look after him, and if he can't do it himself, Bruce will just have to take up the reigns for him.

Tony Stark and Bruce Banner (c) Marvel Comics.