I'm glad we've left the days of "you complete me" behind us and it's finally in the cringe museum where it belongs. I think a relationship eventually runs into big problems when there's codependency. If you can't cultivate the love within yourself in solitude, you will never be fully satisfied with the love you receive externally. People are also fickle. They change their minds, hearts and desires. They get bored, they get sick and they pass away. I believe strongly in the truth of impermanence which is part of the Buddhist philosophy. It's more of a guideline of how to deal with changing circumstances by learning about the strengths within you by embracing surrender and letting go of attachments.