Why is it that when we love others, we feel the need to be in a relationship with them?

Because, it is the relationship that becomes the foundation of a future that they can have together, as a unit, to perhaps grow into having children of their own. (After all, that is a biological calling, even if people do not choose to take this path.) Sharing into each others good and bad and committing to it.
It is certainly not about possessing someone. It should not be, at least.

If people love each other, why would they want to keep their options open to begin with? Why can they not be independent within the dependency of a relationship? That is selfish too. If one wants to keep their opportunities open, the individual relationships tend not to find the depth that a relationship in solidarity finds. And that is fine, if both people want the same.

So I'd suggest people find like minded people to be with.

The whole point is to start a family. We are social creatures and do well working together instead of on our own.

I don't want to own my bf, but I don't want him with anybody else. Maybe that's selfish. I don't care.

Because when you are in relationship, (at least if both want it)it's because they really consider special... unique... You don't need other romantic interest because there's love and you stay in that relationship because you love them and they love you. Not like a slave.

I like Aidolovemostofyourthoughts' comment.

So yud marry a whore 🤔

That's true love. Yes.