Hulk (2003) (part 9 of 13)
The next morning, Betty calls her father, and totally sells Bruce out. She tells General Ross exactly where to find him, for reasons that will never be explained. Cut to her fixing Bruce a nice plate of psychobabble.
Bruce: How could it? We designed them to react to physical damage.
Betty: Emotional damage can manifest physically.
So, now all the pieces are in place. The Hulk is the Hulk because his daddy injected all kinds of animal genes into himself to improve his immune system. These genes somehow got passed onto his son, who later inhaled some magical healing nanites that are powered by gamma radiation, and which make everything explode.
However, because of his increased immune system, the nanites don’t make him explode—they just heal him very quickly, except whenever he gets upset. In that situation, they activate some kind of Matrix-like “your mind makes it real” bullcrap and cause him to turn all green and giant, because emotional damage can manifest itself physically, except he doesn’t transform when he experiences actual physical trauma. This is so much simpler than just having him be mutated by radiation, isn’t it? Isn’t it?