Applesauce

Brain tumors literally affect people's judgement and decision making skills among other things. He literally even says in parts of the story that he doesn't know why he is doing some of things, but he cant control himself and does more things for no rhyme or reason.
The tumor is why, which is why in his late stage, usually people have family looking after them.

Uhhh.... spoiler
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He gets worse, does some more crazy stuff, and it's been a while since I checked it out so im trying to remember, but I think he had like a seizure and was vomiting blood, and his family was there (I think it was his sister and her girlfriend, but i could be wrong). But they find out, the girl finds out, the ML find out, and are like "Oh... it all makes since now. Dude, you need a care taker". So the ML is watching after him after that, and they move and rent a place by the beach.

I'm aware I studied neuroscience before deciding on my degree in university and I read the raws, I was just commenting in frustration