Worth remembering that music really does affect us physically. It's vibrations, which cause vibrations in our own bodies. We are changed by it. Plato wrote this, and very recently contemporary scientists have confirmed this. As for a single song or piece needing to be heard again and again, I can't speak to exactly the same experience, but there have been times when certain pieces have worked particularly well for me. Following my dad's passing, when I was laid up w/ bronchitis and just generally in a low state, I found the Brandenburgs (in a particular performance) very uplifting, almost literally. I suspect it was in part that these pieces are both joyful and easy to listen to, and yet amazingly rich and complex beneath that surface, and thus one is both comforted and engaged by the listening: it does not wear you down, and it also doesn't get old. Back in high school I could count on certain Dvorak chamber pieces to bring me out of the dark place of teen angst. Pieces also take you through a sort of process- tonal music (and certainly some non-tonal pieces) are very much about tension and release- so going through that listening process is indeed like walking through a kind of therapy. In talk therapy, there's often some sort of crisis before catharsis; in massage therapy there's the good pain before the release of the muscles; in the endorphin release of exercise you need to exert to get that good burn and post-workout peace or high. I'm guessing your piece simply does this for you very well, and it fills (or filled) a need for you, and your body/spirit knows it and craves it. Sometimes the subconscious is pretty smart, and is worth listening to. :)
