How can love last?

Oh my sisters and my brothers, is it choice which makes us unhappy. Like Imelda Marcos with her countless shoes, too much becomes an illness. And so with our choice of partners, there is so much gorgeousness out there. But each one in some way flawed, or wrongly specified, or underperforming on some parameter, or liable to sudden malfunction, or in breach of the Trades Descriptions Act for designated soul-mates, or scuffed or damaged, or too loud, or too faint; or just something you can’t quite put your finger on.
Seems it wasn’t always like this.
Lawrence Stone in Family, Sex and Marriage in England, 1500-1800 puts happy, or at least tolerable, marriage-type partnership during most of that time down to just two things: one, late marriage; two, the early death of at least one of the partners, usually the man (Stone confines himself to heterosexual relationships, we needn't)
Romeo and Juliet mislead us. Most men couldn’t afford to get married until their thirties. And then they mostly died in their forties. Their widows tended to be younger and still bouncy. And merrier - there were a lot of unattached young men around.
Stone reckons ten years or so was about as long as a couple had to be able to stand each other before death superseded the need for marriage guidance.
So how to account for a long and apparently happy partnerships today, for Golden Weddings and those Darby and Joans in their nineties, aliens to our world, but still apparently happy in theirs?
At a party back a while there were a few of us in the kitchen, discussing big rows we’d had, the depths of marital or quasi-marital excess and violence, saucepans so badly dented they needed replacing, doors requiring serious carpentry, the thrown egg still on the wall a week later because neither launcher nor target saw it as their role to remove it - how angry must someone be guilty of making you before you throw an egg at them?
Among all this battle-scarred rabble was one sweet faced angelic girl who said, “Me and John have never had an argument.”
“What?” We said it in unison, and a little too loudly.
“There never seemed anything important enough to argue about.”
Within three weeks, with hardly a word no doubt, the angel and John had parted for good.
If you are together for a long time, and you are not zombies, then this is probably going to happen. You are each going to grow apart from the other, and after seven or twelve or however many years, you are going to really fall out, and go your different ways, and hate each other. And then with luck you are going to talk, and go on talking, and gradually discover the person that the other has become, and it will all be very painful, but if you are lucky you will like, maybe love the person the other person has become, and vice versa, even maybe love them better than the person you liked or loved all that time ago, if you can quite remember who they were.
And that is maybe how it works; a bit like an arranged marriage, only not at the beginning, at half time, or the first interval. I’m not advocating arranged marriages, though I have relatives and friends for whom they have worked very well. But I’ve never quite believed the slogan “What people want is more choice” either.