Why fruk? All they do is fight!!! That's exhausting and toxic
I know I took a long time to reply to this but I wanted to have time for it.
There are many reasons, from the usual rivals to lovers to the doomed lovers and gravitational forces that keep clashing to the lovely reason of face family (children), but I'll admit, I don't think face family is a thing. While I do love it, I don't think it's realistic at all in nation aus, they're too young and focused on ambitions and other matters; besides, they'd never join to raise the kids because France is not a parent, only when kids are older and interesting; England mellowed to kids with age, but he was a brat himself, America was the one pulling the strings like only a child does, tbh.
I could give you many reasons, from their personalities to the fact that there's not a single soul that knows them better than the one who witnessed all your failures and victories, but I decided to address the point you make about it being exhausting.
It is, but at the same time, it's invigorating.
As someone who comes from a house of maniacs who always fought, I'll admit I don't understand it that well sometimes, but there's a point to be made here, and that is: love is not enough.
These two are really good to showcase this point because they do love and hate each other like no one else, and that's not enough. I think that's the main appeal of this ship tbh.
Their love is not enough to be together, not enough to be apart, not enough to ignore the other, and not enough to lower their guard and properly try. And isn't that the most humane and real fact? I do gravitate towards them in all the ways they're portrayed because of it. You can make them sweet, you can make them less proud, and you can make them human and fragile, but in the end, they're who they are and will always fight. But while they do fight a lot, they are used to it; they're desensitised to it, even if it looks toxic or abusive. They have a mutual understanding that no one—them included—can understand properly, but it works for them, which is enough to make them crave this push-pull relationship.
They are ancient, they are used to a certain level of cruelty, and their morals are flexible. Besides, they have thick skin and don't care about certain words or violence. They are and always will be opposites, that's how they operate, their role in the world, and how they feel comfortable acting, because that's what built their identities up and carved their name in history.
(BTW if their hate was really deep and vicious, they would use Canada and America against each other, and the lack of shippers of frus and engcan tells me that maybe people do get that deep down they don't hate each other.)
They're also sadomasochistic in complementary ways. They both enjoy causing each other pain.
As I see it, France is more of an emotional sadomasochist; he finds real pleasure in humiliating people, and tears can be aphrodisiac in beautiful individuals, so it's a treat when he can't wrap someone in his fingers, and England doesn't give him that pleasure. He also enjoys feeling pain; he is that one individual who abides by the rule that the most pleasure can only be obtained with a certain amount of pain, and he does love teasing but can't handle it himself. Who else can fulfil his inner desires? Even those he will never admit he has? England will because he sees who he really is underneath his well-made web. England can make him feel his own poison, and as much as it's infuriating, it makes him addicted to more, and he won't deny himself that pleasure. (No pleasure goes to waste for that guy)
England craves something more physical and cathartic, so he enjoys when the other presses and doesn't give up when he doesn't give them the pleasure of showing his pain (he is too proud, really). He also has a sharp tongue that hurts people without his intent, so he needs someone who loves himself enough to not break with his vitriolic words (France loves himself too much to believe his words). England also enjoys hurting and being hurt; it can be relieving and usually clears his mind, as little does. Besides, it's a win-win situation because France also looks his best trying to contain his tears at his feet and is amazing at teasing England until he breaks down without taking advantage because he prides himself on being a good lover above all, even his own desires sometimes, which can be reassuring to someone as jaded as England.
There's also the fact that he does enjoy playing the game. England enjoys mental games, and this is pretty much an even one; sometimes he breaks, sometimes France breaks, which will never cease to be a motivator to keep it up despite that nasty fact that France enjoys reasons to do things, emotions, and all that crap. France is invested; he finds it fascinating in several ways and longs to be the one to really understand England or make him fall for him, whichever comes first (he lacks self-awareness sometimes for such a self-proclaimed wise nation). And England is, underneath all his bitterness, a little romantic, so he quite enjoys being pursued, so why not? At least the frog is one of the best-looking nations, despite his many flaws. (I will admit I don't really portray England as tsundere as the anime makes him but the fact still is that he will never be open about his desires, will never be dovey and sweet as France wished him to be but France kind of grew to like that part of him too, despite not being cute)
I could also add the fact that they're both dominant. That's another point of contempt but also complimentary; I'm simply in love with the idea that they switch up and France is that annoying dominant bottom who uses England as he wishes, not really realising that England quite likes being used like that (cough, cough, English vices). They never win with each other because they end up unwillingly giving each other reasons to keep up.
France calls it fate, England his curse. Either way, they'll always be connected by history, geography, and even humanity.
I rambled my way around the point, but the thing is, they enjoy this game, (It's their game) and yes, it can be tiresome.
Sometimes they really need to step out and give it a rest, but they're both confident enough to know it's temporary. They'll cool down when things get too bad and then call each other to gossip about something and find each other in the same bed in a single day, not really knowing how but thinking that yes, they actually missed that annoying guy.
Personally, I don't see them fighting all that much, but they do know where to poke to provoke, and France is usually the one poking because he needs drama in his life, and England gets bored easily, so he needs a push to keep lively, but they do know where to stop. (England goes along with it because it suits him and everything is boring; he keeps thinking too much, and this way he's engaged in something that's not self-destructive for once. France just lives for the movie life the drama queen)
So, they don't fight as much as you'd think; it's mostly for show because they need to be the antagonist force, and it's mostly about their differences in handling things and views that usually don't stray all that far, so it's reserved for their nationhood things. The fact that in Canon France demands to fight/argue with Germany also adds to my view, because who will counter him now? (He needs that voice of reason/oposition/ there; it's just unnatural to not have it there.)
So yeah, it can be tiresome, but in a comforting way. Is that a good reason to ship them? Maybe not, but there's no doubt they make a compelling case. Be it hate or love, they're not indifferent to each other, and that's a fact.
Sorry for the long rant but I've received like 3 questions about fruk and I joined most points here.