I think a lot of people (myself included) like to have a partner, and accept that there will be pros and cons to this and there is a level at which the pros outweigh the cons.
This doesn't have to be at a conscious level - you meet someone you get along with, there is a spark, you start dating, and your lives become entangled and while there might be not so good things about the relationship there are also good things and it is harder and more painful to end it than to continue, plus the uncertainty aspect.
I met my 1st H at uni, we settled down into a life together and were happy for a long time ... until we weren't. We had small kids and tbh I'm don't think I recognised my unhappiness as such and would never have ended it myself as I wouldn't have wanted that for my children. And I'm still not sure that it wasn't just the stresses and strains of small children.
I got together with my 2nd H and while there were a number of issues, as there often are in post divorce relationships, the two of us together were completely right for each other and it made me see that I'd been low level unhappy for a while with my H1.
DH used to say I was his soul mate, and I used to tell him that was a load of rubbish. But my heart lifted when he was near, when he entered the house etc. I felt lighter and happier with him around.
I'm now seeing someone (DH died) and I'm maybe settling - but I'm doing this consciously.
I've spent a lot of time considering the pros and cons, and he is a good man, whose company I enjoy, who makes me laugh, who is kind, he pull his weight, the sex is good etc. But he isn't everything, and I probably wouldn't have known that if it wasn't for DH. However he is an awful lot, and tbh I'm happy with that and I'll find the missing bits in other parts of my life.