For example, there are clear health benefits associated with long term relationships and marriage. Some antisocial people would like to believe that that doesn't matter because they can replicate these benefits if only they eat clean, exercise, abstain from alcohol, etc. Well, turns out, there are health benefits to LTRs that cannot be replicated otherwise. Your theory is one possibility. But simply eating clean, working out, getting a lot of sleep, drinking water, and having clean bills of health and good blood tests isn't enough. You will still die earlier than someone who's not single. You won't have the discipline to maintain this lifestyle until the end of your life.
But you can't enjoy the benefits of singlehood and the benefits of LTRs at the same time. This is controversial. But you can't have it all. You can't optimize your life for everything at the same time. We're disgusted at the idea that we may have to pay a cost or sacrifice something. In this case, sacrificing the freedom of choice and the flexibility that comes with singlehood for the health benefits of LTRs. But no. We want the freedom of choice, the flexibility, and the benefits of LTRs.
Similarly, you can't become rich quickly without taking an incredible amount of risk that will in all probability leave you worse off. Other than luck, the true and tried path to long term wealth is to grind for decades at a job or a profession that brings money. Being a doctor, or a FAANG SWE, or starting a business. This all takes hard work and patience. But some people would rather try their hand at get rich quick schemes with cryptocurrency, options trading, or other scams. Very few succeed. That's luck. Most fail.
This assumes the reason people in LTR's live longer is because of LTR's, and not that healthier or more fit people are more likely to score LTR's. IME people settle once they think a relationship is locked in and let themselves go.
On the other hand, when you're a couple, you might be tempted to let yourself go, but at least there's a chance your mate will refuse to go. Since our behavior is driven by environment (a la we observe the norm, and conform) you'll resist temptation and stay healthy. Habits are easier to maintain when you're not alone.
This alone might give couples the edge.
Fascinating hypothesis. I wouldn't be surprised that shared diet, living space and activity leads to a converging of gut bacteria. But how would this lead to health and happiness?
I believe they say, we have more bacteria than cells. Makes me wonder who is hosting who. The point is, those bacteria matter.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6458105/
The paper also points out that because of all the concordances, couples tend to share the same downfalls, not just the good things: disease risks, bad diet, bad sleep, emotional turmoil.
Seems to defeat your theory.
That said, and not to get off topic, my theory is we'll eventual discover that there is a gut microbiome component to couples living longer than singles. For much the same reason studies say people with pets live longer. Yes, there's the emotional companionship (that impacts the physical) but there is also the exposure to another's bacteria.
Again, just a theory. But the more I read about "the gut" the more important and influential it's becoming.
*based on readings, often here on HN, I'm not a medical professional