In each movie the Hulk is in, he is found and he is being asked for help. Then why does he always leave the town?
Hulk is a threat to any living being next to him. At the end of every movie there is a war or battle, etc. He feels bad about himself after incidents like that, so he leaves. But the truth that Hulk is a hero and saviour of humanity doesn't change, so he comes back to help. But he can't and won't attach to anybody because of his condition.
I think you might be mixing up continuities. The Hulk saved Iron Man in the first movie, and shows other signs of lucidity between the two movies. Also, he was not "found." The World Security Council (and Fury) had never lost track of him. And the whole leaving town thing? I think that just harkens back to the old TV show; nothing to do with the current MCU.
Hulk is based on comic character known for his anger being out of his control. In fact, intially he converts into Hulk only when he gets angered by someone. He started controlling himself in Avenger first time which leaded to many questions e.g. this and other . So Bruce Banner prefers to live away from city either alone in forests or some peaceful tribes where people will not bother him.
Bruce Banner and Hulk are almost two different entities like Dr. Jekyll and Mr. Hyde. Hulk may not be evil as Mr. Hyde , however his out of control anger results to lot of destruction. So Bruce Banner needs lot of confidence to live in city and other side , how much he has control on Hulk, someone will find way to piss of him by experiments or other means.