>>3344553tldr: to gain control of his own misery, so to speak.
ritsu and mob have been growing apart for a long time. ritsu wants mob's powers and finds it unfair that mob's never done anything to deserve them; at the same time, he loves his brother, and yet, he's also afraid of him. i'd imagine his insistence on making sure mob's well-being is safe is for both mob's sake and his own sake, too, because of what ???% did to him when they were younger. mob doesn't give room for equal and honest communication either, because he isn't willing to confide in ritsu nor does he pry further when ritsu doesn't want to talk about his own problems. not to mention, everyone has high opinions of him: they think ritsu's perfect, but ritsu doesn't think any of his traits are worthwhile because he's devoid of psychic powers. i don't think it's revealed why he wants psychic powers other than it being simply envy.
his actions during the big cleanup arc stem from the all the aforementioned stress, shifting from being passive toward them to finding an outlet and some means of "changing", even if it means he has to partake in his own anguish.
my breakdown of it is utter garbage, but hopefully ya get the gist
