I've always been a strong believer that we're naturally polyamorous or have relationships like chapters of a book—basically all mammals are. But we socially protect each other as well, resulting in our phenomenonally high divorce and separation rates which just reflect the two conflicting natures—pulling the trigger on something that someone realised long ago, but couldn't bare to do it for the sake of protecting the other.
If you add on those that stick around just 'cause, we have the majority of people expressing statistical incompatibikity with "mating for life", almost as though it's not how it were meant to be.
Those that overcome this see their relationship as a partnership toward life. Long after the attraction of romance is gone, the romance of being with someone you work so well with is the new attraction. You love them because of what they represent; compatibility, a PIC; not a partner for life but a life partner. It's then that love becomes admiration instead and a relationship is at its most fulfilling.
But it can all change one day. And that's absolutely fine. You don't live for another and they shouldn't live for you. Paths in life should only ever he parallel, never intertwined, that way a person can live their life as it's the once chance they have to. There should never be animosity toward someone wanting to be free or yourself wanting to be free. Because that, is not love.