If not we will fall victim to our own or their baggage and constantly be subject to its festering wounds. For some of us its childhood trauma and some unresolved issue with a parent who is now deceased. For others it will be abandonment issues and feeling insecure in our skin such that the partner becomes a panacea that never does the trick.
The other person doesn't make us happy but we are lulled into this illusion via the advertisement of romantic love as a cure all from more than just loneliness. With time that promise feels more like a lie we should have been able to read. It is we who make ourselves happy or miserable and the partner is an addition who can bring richness or tragedy with the understanding that compromise cannot be avoided. Even if you have your act together, their trauma may add complexity which forces you off balance and breed regret where at the outset there was only the euphoria of burgeoning pheromones.
We come to understand this difficult balance with time and many of us are currently living in situations where we suffer. But if at least we detach and realize that we cannot shoulder all of the partner's ills we will be one step closer to internal balance.
To a great degree romantic love is an illusion which becomes evident with the passage of time. But if we are fortunate enough, we will have set down enough roots of familiarity of experience to make separation less palatable than the alternative.