Why do we form relationships?
There are many reasons, but they can be summed up by the concept of meeting needs. And how many needs you have (and their complexity) may determine what kind of relationships you choose to have.
Let’s say you have 100 need boxes to be filled. A typical primary relationship fills, say, 75. It’s then up to you whether the remaining 25 are important enough to be filled.
Monogamous people say, “Nah, 75 is enough. I can live with the empty 25, and maybe over time, my partner will fill some of the rest.” Polyamorous people find others who can fill at least some of them.
What if you don’t have a primary? Then, one partner may fill 30, another 25, etc. How many partners you take depends on how important it is to have the other boxes filled — recognizing that some will always be empty. The specific array of partners and changes in the lineup determines which boxes get filled at any specific time.
A potential partner might appeal because they fill a few high-need boxes, and so we’re willing to put up with complications like distance or some incompatibilities to get that benefit. People who have a really big need box for companionship, stability, and/or touch may be extra motivated to get a primary.
And yes, some people decide to fill some of their own boxes — or shrink some of them to make that easier. Of course, some boxes might be more resistant to shrinking than others.
But every time we take on a partner, there is a calculus of what boxes they will fill, whether we can manage what’s left — and how.