>>117001221As a boy, he was constantly in danger. Being in danger caused a body reaction of essentially always being in 'action mode'.
While in 'action mode' the body expects things that are never there, but reacts the same way as if it was there. Steven getting pinked and constantly having mood swings was his own body reacting to a threat that wasn't technically there until he realized that the threat was in fact, himself.
Given the episode where he starts swelling, his body was already trying to react to a threat, but he didn't understand why at first, and then by the 11th and 12th episodes, he saw the threat was himself and his body had failed to react in time to the threat, causing him to become a monster.
The cracks in his skeleton should've been a clue, as the cracks were healing pieces that had already swelled and his body was, for the most part fixing itself.
He didn't have any pink episodes after that, up until he had convinced Jasper to teach him how to be a Diamond. That's how 'Chad Steven' was made, a body that was made specifically to control the power that he gotten used to. Yet as he continued to go about his 'average' day, the power he had started to once again go haywire, his body literally reacted as if it was being killed, so it turned him into something that could:
Not be hurt
Not be stopped
Not be reasoned with
Not be confronted with conventional means
His monster form was literally an antibody trying to kill a disease: His powers
