(Epistemic status: quick speculation which matches my intuitions about how social things go, but which I hadn’t explicitly described before, and haven’t checked.)
If your car gets damaged, should you invest more or less in it going forward? It could go either way. The car needs more investment to be in good condition, so maybe you do that. But the car is worse than you thought, so maybe you start considering a new car, or putting your dollars into Uber instead.
If you are writing an essay and run into difficulty describing something, you can put in additional effort to find the right words, or you can suspect that this is not going to be a great essay, and either give up, or prepare to get it out quickly and imperfectly, worrying less about the other parts that don’t quite work.
When something has a problem, you always choose whether to double down with it or to back away.
(Or in the middle, to do a bit of both: to fix the car this time, but start to look around for other cars.)
I’m interested in this as it pertains to people. When a friend fails, do you move toward them—to hold them, talk to them, pick them up at your own expense—or do you edge away? It probably depends on the friend (and the problem). If someone embarrasses themselves in public, do you sully your own reputation to stand up for their worth? Or do you silently hope not to be associated with them? If they are dying, do you hold their hand, even if it destroys you? Or do you hope that someone else is doing that, and become someone they know less well?
Where a person fits on this line would seem to radically change their incentives around you. Someone firmly in your ‘worth keeping’ zone does better to let you see their problems than to hide them. Because you probably won’t give up on them, and you might help. Since everyone has problems, and they take effort to hide, this person is just a lot freer around you. If instead every problem hastens a person’s replacement, they should probably not only hide their problems, but also many of their other details, which are somehow entwined with problems.
(A related question is when you should let people know where they stand with you. Prima facie, it seems good to make sure people know when they are safe. But that means it also being clearer when a person is not safe, which has downsides.)
If there are better replacements in general, then you will be inclined to replace things more readily. If you can press a button to have a great new car appear, then you won’t have the same car for long.
The social analog is that in a community where friends are more replaceable—for instance, because everyone is extremely well selected to be similar on important axes—it should be harder to be close to anyone, or to feel safe and accepted. Even while everyone is unusually much on the same team, and unusually well suited to one another.
Meteuphoric 
1) One difference between objects and relationships is in who is responsible for the repair. If your car is broken, then you are the only person who will fix it, and you definitely can fix it if you spend enough money. But if you have say an alcoholic friend, it’s not your responsibility to help them, it’s not your fault if they continue to be alcoholic, and whilst you can put a lot of time and money in to helping them, you cannot force them to change, it has to be their choice. So, unlike a car, “fixing” a friend is far less easy than replacing them.
2) Another difference is that you would hire a mechanic to fix a car because they have skills that you lack, but for some reason when it comes to relationships people generally ignore this logic and instead of hiring a skilled counsellor they try to fix it themselves. Again, this results in relationship fixes being more difficult.
3) Over time, applying Bayes’s Theorem would probably cause you to stick with the sunk cost. You bought the car assuming it was reliable, and you were wrong, so what makes you think the next car will be more reliable? The more often you observe broken cars, the more you’ll be inclined to stick with what you’ve already got, and the same would be true of relationships. Note that many more people have a second spouse than a third, perhaps because they have learned that changing spouse hasn’t solved all their problems. Which leads to…
4) There may not be a better alternative, even in a situation where everyone is well-suited. You’re never going to be able to replace a broken car with an indestructible car, or find a flawless friend to replace a flawed friend.
Interesting! “The social analog is that in a community where friends are more replaceable—for instance, because everyone is extremely well selected to be similar on important axes—it should be harder to be close to anyone, or to feel safe and accepted. Even while everyone is unusually much on the same team, and unusually well suited to one another.” <–Does this prediction seem borne out by the data? I thought the conventional wisdom is that birds of a feather flock together, and that homogenous communities can be very tight-knit. But maybe there are other factors that account for that, like ingroup/outgroup stuff.
The evo psych concept of welfare tradeoff ratio seems related to your thoughts here. It’s essentially how much of your welfare you’re willing to trade for someone else’s. If your WTR for your sister is 1, then you value her welfare exactly as much as yours. There’s been some good research into the “internal regulatory variables” that drive and update WTRs, such as if the person is related to you, in a trade relationship with, very formidable, very sexually attractive to you, etc. It’s more of a precise description than an explanation, but I still think the clarification is interesting and helpful.
Click to access DeltonRobertson2016.pdf
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Thiis is a great post thanks