I think (serious dysfunction aside) eleanorigby has the right of it. I know that, although I love my family dearly, the longest I can spend cohabitating with any of them before I loose my effing mind and have to yell at someone is ten days. We’ve conducted tests to determine the duration, actually. The limit is about the same for all of us, so we tend to schedule family visitations to less than that span of time.
Also, my brother and sisters and I can spend longer with each other than with our parents before the meltdown happens. Not because our parents are difficult to be around - just because all of us are adults and used to managing our own households. We are definitely no longer in the habit of not being at the top of the household pecking order. I think everyone lives by a “my house, my rules” sort of philosophy once they get out on their own - if you’re married, it’s “our house, our rules” but everyone is generally at minimum a (if not the) primary decision maker of how things are in their own house once they go out on their own. Having to follow other people’s rules is… grating after a while once you get used to your own rulemaking. Not even for major things - but for small things, too - like shoes on or off in the house, where the potato chips are kept, portable phone always back on the charger after each call or does it migrate with you, dishes done immediately after use or in batches, pets on furniature or not, do you flip channels during commercial breaks in your TV show or just wait for the show to resume, etc.
In a lot of ways, family is harder to deal with this way than other people you might be visiting, because you have the whole endless “I’m a guest, but s/he’s family! I’m a guest, but s/he’s family! I’m a guest, but s/he’s family!” loop to deal with also.
I love them, and even like them sometimes, but viewed objectively, I think they actually are difficult people.
Mom-- very controlling, sees the world in black and white.
Grandma-- Enjoys a good argument and if one’s not happening, she’ll help it. Also, if someone tells her God said it, she believes it, that settles it.
Uncle 1-- An extreme bigot, very pushy and opinionated.
Uncle 2-- He’s either very religious and easily offended or very stoned and anything goes. Treat with caution.
Brother-- A good guy with a lot of problems he had a hand in creating. He’s not difficult to like, but he’s difficult to help.
I’m no easier for them to deal with.
I like my parents a lot, and the biggest problem at the moment is that I have to practically drag them out here to get them to visit. They’re the ones who find spending time with us much more difficult than my husband or I do. We’d really enjoy having them visit a little more often if it weren’t such a problem for them!
The biggest problem when they actually are here, though, is their issues with each other. On an individual level, they’re both fine. But after 42 years of marriage, they really can’t stand each other most of the time, and that gets very difficult to cope with!
The in-laws are great when they come here, in part because they have dogs at home that they don’t want to leave for long so they’re only here for 2-3 days at a time, and anyone can be nice for that long! 
When we go there, though, the dogs are an issue, and I get antsy to leave within 24 hours. Aside from that, they’re fun people to be around, and they’re really laid back so when we’re visiting, if hubby and I want to go off and do something on our own without them, they’re ok with that and don’t get all upset/offended as some in my extended family would. The laid-back thing really helps just in general to make it easier for everyone to have a good time. My family could learn a lesson or two on that front!
True to a large extent, but then, how do you explain that people who have to spend time together (children/parents, spouses in arranged marriages) usually manage quite well? Because they *really *have no other option. In traditional societies, only death, or a divorce with enormous negative stigma, can break up such an (arranged) marriage. In families, parents and their little kids have to adapt to living together.
It’s only when we are adults and living on our own, inother words, when we have an alternative to living with our folks, that we can allow ourselves to feel the irritations we had to suppress back when we were kids and going our separate ways was not an option.
So, in short, we’d feel this way about anyone we had to spend time with, don’t need to spend time with anymore, and still spend time with anyway, if that makes sense.
That probably explains a lot about puberty, as well.