The purpose of a relationship is not to be loved perfectly, or forever. It is not to have our every whim and wish met and fulfilled. It is not to be completed, or to have our minds and hearts fueled by the hormonal stimulation we think is the feeling of love. The purpose of a relationship is not the Universe's way of saying "you're worthy, and here's someone to prove it."
The purpose of a relationship is to see ourselves completely. It is to see the parts of ourselves that we are otherwise unconscious of. The purpose of a relationship is to infuriate and overjoy and destroy us, so we can see what angers us, what thrills us, and where we need to give ourselves love. The purpose of a relationship is not to fix us, or heal us, or to make us whole and happy, it is to show us where we need fixing, and what parts of us are still broken, and perhaps the most brutal of all: that nobody can do this work, or make us happy, but ourselves.
We choose to love people who cannot love us back to teach ourselves that we are, in fact, worthy of being loved back. We choose these people because they represent the parts of us that we don't love - why else would we waste our time on people who don't return our affection? We choose to love these people because they are the only ones with whom we share an intimate connection deep enough that it can awaken and illuminate the darkest corners of ourselves, and they are the only ones who can leave and let us do what we are here to do: resolve and actualize and heal them on our own.
It is not the nature of love that people struggle with, but what it is designed to do. Most of our turmoil simply comes from never having been told that love will keep breaking our hearts until they open, and that we will be the ones throwing ourselves in again and again.
Our life partners are the people who come after the love that opens us. Our big loves are the loves that emerge after we think we've lost them already. They come after we're ready, after we've already cleared out the damage and debris, only after we've learned what it means to love ourselves. It is in this we realize that love is sharing what we already have, not relying on someone else to give us something to supplement. It is in this we realize how crucial it was to love the people who could not love us back. They were never meant to, and the rest only depends on how long it takes us to realize this.