At best, two people make a new branch of society, begin a new society, found a new tribe. It is a way of saying yes to your own way of being, that you are willing to advocate it for someone unborn.
David Benatar challenges the idea that coming to be is intrinsically positive, in Better Never To Have Been Born. I'd challenge his conclusion by saying, we are built on a tower of people who said yes to passing on their inheritance, to their own conditions of being, augmented by what they saw in the genes and socio-cultural skills/vision of a partner. To form a lasting partnership to raise children, is a declaration of hope that they can be part of the journey towards better being - or it should be, to bring children into the world in good conscience.
I jump to conditions involving offspring, because I take that to be implicit, in seeming to exclude consideration instead of finding deep friendship or comradeship. And the apparent disinterest in seeking someone to experience sexual pleasure with for it's own sake, as being it's own motivation.
The poet Rainer Maria Rilke said
"But what you love to see are faces that do work and feel thirst.
You love most of all those who need you as they need a crowbar or a
hoe."
Personally I think that is better, than being dome kind of free-floating brain capable of total self-sufficiency, that only chooses partnership with another having already attained 'satisfaction'.
But I just don't think there can be one recipe for everybody. Just try to make joy when you can, is the only advice that matters.
"Praise tinker and saint, and the rose that takes its fill of
sunlight, though a world breaks."
-from The Storm, by George MacKay Brown
“And what would humans be without love?" RARE, said Death.” ― Terry
Pratchett