A friend thinks that as we get older we get more set in our ways, and that those of us who live alone become more and more accustomed to being alone, and thus less and less likely to attach and become domesticated. I don’t want to believe her, but her point is well taken. For we have our routines, systems, and stuff (baggage both material and emotional) worked out to varying degrees, and perhaps we’re less willing to compromise and rearrange ourselves as necessary to fit another sentient being into our lifespace.
Sometimes, though, I think the coupled person is the more well-rounded. It’s precisely those compromises and negotiations, and the resulting sympathy and empathy, which make up intimate relationships that result in a stronger individual. We are social animals. We need to touch and be touch. I don’t actually sleep all that well with another body in my bed, but I’m warmer, more comfortable, and happier when I'm "sleeping" with someone. It’s just a pity (for those of us who are such klutzes at it) that coupling is the only real option to singularity; hence the vital importance of friendship for the uncoupled. Hence, on the malign side, the suffocation of families, the creepiness of clans, the insidiousness of nationalism.
There are exceptions, of course. And then there's polyamory.