I mean to focus specifically on long-ish-term relationships and not so much on casual sex, though if you have insights regarding the latter, feel free to contribute.

Basically, there are lot of single people in this world who would rather not be. Some of them might have negligible appeal or insatiable standards, but others could very nicely be matched into psychologically, sexually, socially and logistically compatible couples (I'm not talking about Romeo/Juliet or Odysseus/Penelope here; just normal, fulfilling, relatively convenient relationships). The reasons they're not together are purely incidental - maybe one of them didn't install the right dating app, didn't walk up to the other at the bar, made a bad judgment call when evaluating the first date, or simply failed to get into an accident where they both fall over each other in the hallway and drop their books and papers all over the floor.

Now obviously it is a market failure when that happens - they would both be better off if only the Invisible Hand had brought them together, but it somehow didn't. But do you think it happens a lot? Maybe the market really is good at matchmaking with only a small number of very unlucky exceptions, with everyone else truly being a non-catch in their current community; or maybe there are thousands of people experiencing loneliness when they really don't have to. If it's the latter, then is there a way to fix this problem?