There is a vast variety of human relationships can be characterized by many dimensions such as duration, intensity, depth or quality such as romantic, platonic, familial, intellectual and so on. The quality or positive significance of a relationship can be judged according to many criteria. Sometimes we can learn something valuable from bad relationships. Bad or painful situations in general can teach us a lot. But as far as suffering is not a goal, the best relationships are those with the most upside and the least downside.
What I've noticed is that sometimes the return on investment is overwhelmingly good when the duration of the relationship is ultra short as in measured in minutes to hours. And I'm not necessarily talking about what you're probably thinking about like one night stands with hotties who who wouldn't normally give you the time of day but who find themselves sufficiently disinhibited by circumstances (such as being in a strange environment and having consumed the right amount of alcohol) to want to jump in the sack with you.
I'm talking about things like a simple conversation with a stranger in public transport. Some of the best conversations I've ever had have been with strangers in that kind of situations. When you both know you're extremely unlikely to ever meet again, there is zero downside. You can quite freely express your opinions on anything once you've broken the ice and checked you won't be stepping on a minefield when you direct the conversation to a particular area. I've had countless conversations better than even with my old friends with perfect strangers I've only interacted with briefly and ever will. Except for the time spent, there can be a significant gain in return for zero pain.
The problem even with actual friendships or other type of regular interactions with particular people is that there is no knowing what the future holds for you. You could have a fallout with your friend in the future and if you have trusted them with information that could damage you in any shape or form you could suffer damage. Also, it is nearly always the case that the better you know someone, the more aware you will become of the limits of their discourse. The deeper you dig and the more breadth you cover, the more likely you are to discover that some of your deeply held positions could annoy the hell out of your best friend. Yes, you can still be friends and manage your differences but the only way to do that is simple avoidance. You just don't talk about the stuff you consider important and disagree on too much. This applies to everyone, including yourself. Regardless of how well aware you are of them, you have a list of opinions you won't tolerate, too!
The most complicated relationships we have are familial or romantic. If everything works out, the gains can be large and long-lasting but there will always be an investment required, usually a sizable one. The potential downsides are massive as well. It's not for no reason it is said that families are the worst or that everything is permitted in war or love.
Both familial and romantic relationships can end up costing you your entire life if things go awry sufficiently badly. If we were not animals shaped by evolution to have powerful drives leading us to engage in this type or relationships, I have to wonder if anyone in their right mind would actually do so. They do not exist to make us happy. They can, of course, and often do to a certain extent. But they exist for the continuation of the species, not for our amusement. It is perfectly sufficient for nature for us to get the job done. Our happiness is not even an afterthought.
The collapse of marriage and birth rates and the rise of singles as a demographic in the developed world should surprise nobody. I think any notion along the lines of this happening because evil capitalists have got us addicted to looking at tiny screens is hogwash. If relationships were as rewarding in relation to the toll they take as they are made out to be, people would be seeking them out in preference to any of the numerous activities and distractions our extremely advanced and technologically sophisticated civilization affords us. It turns out that in the most extremely urban and technologically advanced societies like Japan, people are the most likely to eschew traditional relationships. I think the Japanese herbivore men's response to the prospect of becoming a salariman who spends his live slaving away at the office under the yoke of his boss only to find upon retirement that his presence at the home he has paid for is unwanted by his wife who then proceeds to kick him to the curb is a perfectly rational one.
I think we're going to see more of the same going forward and the developing world will be joining the developed world in the megatrend described above as it becomes wealthier. The only exception are conservative religious communities who live in isolation from mainstream society to one degree or another. But there is not a single country in Earth, regardless of religion, that hasn't already seen a fall in birthrate as a function of development.