In the beginning of the Avengers movie, Black Window finds Banner in India, treating sick children. This isn't just a humanitarian endeavor for him. Banner purposefully surrounds himself with injustice - with poverty, with senseless death - so that he could be constantly *angry* at something. Being constantly angry allows him to keep his anger under control - it's not a sudden spike of anger that disrupts his concentration and lets the Hulk out, it's a constant, background anger that lets him decide when to unleash the green beast.