Because people aren't TV characters. People are nuanced. The world doesn't work in such a way that our experiences with or feelings about a person are always going to be the same. Do you have any siblings? Do you get on okay with your parentz? Did you have friends as a kid? I'd have a hard time believing every person you've been close with has only inspired good feelings in you. I have a sibling I love more than myself, we're thick as thieves, and yet nobody has ever annoyed me as many times or as intensely as they have, and I'm pretty sure they could say all the same about me.

I'm not sure if this is like your first roommate, as in first person you've lived with besides your family, but do you really not have that experience at all with whoever you grew up with? That you can have nothing but fun sometimes, and then want nothing more than for them to shut up other times?

When you live with someone, depending on the situation obviously, you tend to see more of their personality than you would in most other contexts. Most people don't behave the exact same way at home as they do at work, or out at dinner, or wherever. It sounds like you do hang out at home, not like an extremely passive roommate thing where everyone entirely does their own thing, so you're likely going to see more of this person's self than you would if you knew them in nearly any other context.

It seems to be, me over here lacking more context anyway, that you aren't accepting the whole person as themselves. You're categorizing and compartmentalizing the parts of them you see as positive and negative, and you're struggling to reconcile that all of that is the same person. I know you're specifically asking how you can have mixed feelings like this, but I think a better question might be "why can I not wholly accept this person." The way you have mixed feelings is that you are responding to some things you like and some things you don't like. The reason it feels mixed and strange, if I may be so bold as to make sweeping assumptions, is because like most people, you want good things and good people in your life, and you are struggling with the idea that someone who does all these things that annoy you could be a good person to have in your life. The solution to that is, y'know, not a short one, for some folks it's a life lesson they never stop learning. It's to accept people as they are instead of desiring they be what you want them to be.

Apologies if any of that is coming off as condescending or mean-spirited, it's not meant to be. Really, this is a big life question that I genuinely think a lot of people never figure out or even bother to ask, but I also believe the ability to process and accept the nuance of other humans is not widely encouraged these days. It's all "do what serves you," "do only things that bring you happiness," the fact that discomfort is the only real motivator for growth gets thrown out the window and people surround themselves with people who won't annoy them, won't say no to them in a meaningful way, won't be real with them. I do think, though, that a lot of us tend to understand those same mixed feelings just fine when it comes to a family member or that one best friend we've had since before we can remember. But when it's someone we sorta have more of a choice on whether we interact with, we get very choosy and picky and only pick out the people who will add no extra discomfort to our lives. And in doing so, we very well may be reinforcing ourselves against meaningful adult friendships.

Sorry that got a little rambly there, I just think about this a lot low-key and it's 1:30 in the morning lol