This one is a little open to interpretation because a lot of people have different ideas as to what it means and how it comes to be

Examples:
Song playing during your first kiss
First song you danced to
Song that makes you think of one another
Song for your "first dance" at your wedding reception.
One day FI came home and told me that he heard a song on the radio that made him think of me

and then he started pointing it out whenever it would come on.
Formerly known as flutterbride2b