Think about significant milestones in your relationship and if a particular song was playing at the moment. Songs that hold a particular memory are strong prospects for a love song, such as the first song you ever danced to as a couple or a song that was playing on the radio when your significant other proposed.
Choose a particular genre of music, or band, that both you and your partner like. Then, go through all the albums together to find a song that holds significance for both of you.
Brainstorm together a song that describes your relationship. Consider the journey of your relationship, where you are started and where you are today. Find a song that lyrically captures what you've been through as a couple.