Bruce Banner worked as a scientist on the gamma bomb project when he heroically saved a young man from exposure to a nuclear blast. Unfortunately, the exposure to gamma radiation changed Banner's cell structure, causing him to transform into a raging green monster (known as the Hulk) when he became angry.
My Opinion - The Hulk, the most powerful super heroes, can't control himself so he will harm everyone around him that's why Bruce Banner tries to stop being the Hulk by testing many medicines to his body but it doesn't work.