Writing about the good in a relationship, the best in another person or the positive impact made by that person is an exercise in appreciation that can be exhilarating. Many poets have longed to capture those intangible bits of the soul on the page and love is often the inspiration for those pieces of the self. Examine what you appreciate about your significant other and how that relates to you.
Writing about the bad or sad elements of a past relationship can help the author to release those emotions and find closure. For example, if the author's love was unfaithful the author could express in poetry the pain and betrayal felt by those actions. Getting those emotions, thoughts and feeling out and expressing them requires an honest look at just what happened and how it made the author feel.
Make the relationship as a whole the theme of the poem. Pick moments and experiences that encompass the variety of emotions felt in the relationship. Summing up a whole relationship in a poem requires the relationship to be broken down and dissected to the core elements. This can help the author get to the root of the relationship and determine exactly what defines it.
Relationships can often be defined in small moments, instances when truth is found or special times when an understanding is reached. Select the defining moment of the relationship. It can be simple, like the realization of love when looking into your partner's eyes or it can be a romantic getaway that was once taken. This should be one of the most memorable moments in the relationship.