can see it so clearly in my mind. mattmello both having ocd. CW: them coping terribly w/ ocd, ocd-related disordered eating
mello primarily dealing with morality-related obsessions, but having intermittent bouts of severe contamination obsessions that he never recognizes and never handles well. he's up all hours of the night trying to fend off whatever is haunting him, trying to repent, trying to excuse himself, so on, so forth.
matt's presents typically as very severe paranoia. he runs in cycles between being far too depressed to scrounge up the energy to be anxious, and checking locks obsessively and unplugging the entire house for fear of an electrical fire. mello often comes home to a bare fridge and days at a time where matt won't eat or drink for fear of spoilage or tampering. it's rapidly changing and impossible to predict.
and they play off each other so badly. matt's a slob, which makes mello's contamination-related anxieties pick up badly to the point he's spending a half hour washing his hands and eating standing up in the kitchen with gloves on. it is not entirely uncommon for him to get to the point where he's seeing infestations where there aren't any. mello keeps strange hours, keeps bad company, and generally praises matt for his paranoia, making him feel validated in it, and in turn making it worse. if mello thinks it's a good idea to check the locks every hour on the hour sleep be damned, then matt's probably right to. as he sleeps less his paranoia gets worse and his compulsions get more nonsensical, tied to superstition and folktales.
matt's also liable to crack jokes and throw barbs that set off mello's moral obsessions. sometimes it's just thoughtless banter and sometimes it's fully intentional. it can leave mello locked away praying until his knees are bruised. they're both far more aware of the other's ocd than their own, and not necessarily against pulling each other's strings to their own advantage.
they love each other, but without outside intervention they're liable to drive themselves into a hole of paranoid obsession, slaves to compulsion and only feeding each other's fears. they didn't always make each other worse like this, but as adults they're so tied up in each other they lose sight of other people, other ways of life that don't beat on the drum of meaningless anxieties.