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."
A couple of years ago I wouldn't have had any problems answering this question but I'm astounded at how confused I feel about this question right now. My first instinctual answer when I saw this was Hermann Hesse - but my confusion set in when I started thinking about what constitutes being a philosopher.