Because everyone would like to understand the big guy as much as possible (and Bruce and Tony are eager to study him as much as they can) Tony and Bruce have agreed that when Bruce lets him out in Tony’s hulk-proof room at the tower, they run tests on him. Tony has yet to be brave enough to try and get any blood or DNA samples off him, but Jarvis constantly runs a variety of scans on him.
This this, they believe they may have discovered the source of the Other Guy’s anger issue.
Bruce typically blacks out just as the transformation starts. Occasionally he remembers snippets of the big guy’s memory, but it’s rare. So he’s been relying on these scans and Tony’s observations to understand the Other Guy. What they found breaks even Tony’s heart.
The transformation Bruce undergoes is, judging by the scans, quite physically painful for both parties, but since Bruce blacks out, he often doesn’t feel it, or at least doesn’t remember. The Other Guy does. Something in his physiology makes him almost constantly in pain. And on top of that, the areas of his brain responsible for fear and the “fight or flight” instinct are always active.
They’ve also learned that, while the Other Guy does enjoy “smashing” things, when he’s afraid he would much prefer to run than fight, and is his most violent when he’s unable to escape.
Submitted by morganblackthorn