It's not so much always wondering if I could do better, as it is having a long list of things I'd like to have in a partner (having an interest in Singularity stuff, having certain hobbies, having certain kinks, and so on and so on). Empirical results so far suggest that a really good match can fulfill maybe 85% of the things on the list, but nobody can fulfill every point, especially since some of the things are mutually exclusive. I'd like to have somewhat with a sciency sort of background for the shared way of thinking about things, and someone with background in the humanities for a way of thinking about things that's different from mine. (One could have both backgrounds, of course, but such people are rather rare.) I like kids but wouldn't want to live with them, so I'd like a partner with kids who doesn't live with me, and for all the usual reasons I'd also like to have a partner who does live with me. There are probably also some other mutual exclusions I'm not consciously aware of.

If I were monogamous, I'd have to settle on a single person and then spend time wondering whether this particular combination of things I want is the one making me the most happy. With poly, I can just look for a combination of people who together satisfy everything on the list. Not that I wouldn't be happy even in a situation where only 85% (say) of the things were fulfilled, but fulfilling more would make me even more happy.

(I don't really draw a sharp line between romantic partners and close friends, and find such a division slightly artificial in the first place. I prefer to just count both of them as members of my 'extended family'.)

If you're always wondering if you could do better, you'll put yourself on a hedonic treadmill that will never make you happy. Sometimes you have to say "this is the person, or these are the people, I love; I'm no longer looking for more".

I'm not understanding this. Suppose that you have numerous friends that you care about: would you have to say "these are my friends; I'm not looking for more"? Would you then not be open to making more friends or meeting more people?

While I can understand the problem of never thinking what you have is good enough, I don't see how being committed to improving your relationships and continuing to find more compatible partners causes this problem.