I love staring at a flame, especially when camping. It's mysterious. It seems like you should be able to capture it somehow to experience it more thoroughly. But it's always moving and changing and a photo doesn't quite do it justice.
In a wonderful documentary I found on Netflix (Music Instinct: Science and Song), pianist and conductor Daniel Borenboim observes, "Every note is a lifetime for itself."
Music is bittersweet because it is so beautiful yet impossible to capture. It is only experienced through passing time. And doesn't life feel that way sometimes? We can never truly hit the "pause" button and sit in any moment, good or bad.
Sheet music is to a song what a photograph of a flame is to a fire. The experience is passing, fleeting. But as songwriters, sometimes a song comes along that makes time feel like it stops. And for those moments, life feels a little more sweet than bitter.