If you've seen Sennit's backstory, you'll know that he was made in a lab, and his creator wanted him to be an instrument of destruction, a monster. He doesn't want to be one, and when someone calls him one he gets upset, and this triggers old memories that he pushed away.

He got angry to a point of massacre, and when he calmed down he ran away because he had done what he was made for, and he regretted it.

He still regrets it to this day, and when he remembers it he gets really stressed out, he has nightmares, he can't sleep.

This causes him to get sick and it takes him some time to recover physically and mentally.

He can't control his urges, so his bf helps him out by calming him down the best he can, and talking it out.

and yes he has PTSD and narcolepsy