Some songs in the right circumstances are like keys to doors to the past, for me, to my personal, individual past - a trip backwards in time where the word "nostalgia" seems like a weak attempt to explain what's actually happening, which is this:
The music keys off a series of tumbling domino memories which come unbidden and avalanche down onto whatever I'm doing or thinking at the time, erasing the present and overwriting the new/old into currency. I step backward into a previous version of me that is so different and pure and incandescent with supercharged potential and anticipation and unlocked grit-your-back-teeth-and-GAH! that it/he feels utterly alien, but I remember being there, then, him. I remember how the smells of those people around me made me feel, seeing them in my mind's eye, through my perhaps unconsciously edited memories of that time. I hear them and the spaces around them, I feel the anxiety and rush of events past. Hormones, surely, but I'm in touch with them now, and the rush of the chemical wave is something tangible, tasteable - it has mass and I carry it. It carries me.
I am transported - not through something so lame as the hackneyed and ham-handed "power of music" but along neural pathways I thought I burned out with injury and drugs and booze and sadness and happiness and new experiences long ago, but the brain abides. It pushes back against the present, be it mundane or thrilling or necessary or all three and says, "this is a thing that happened, this was a time you were in, these were people you knew, and all of it is still in here, locked behind a paper-thin Japanese sliding panel and it can be unlocked at any time by this song." Or not. It's not consistent. The subconscious must be in a mood to cooperate or you'll just get the song and "...yay."
And then, the tumblers in the lock click and slide and roll back the other way and I am left with a desperate need to preserve the experience by using what I know will be inadequate words, as best I am able, but with the hope of crystallizing what it was like, and maybe even sharing the ineffable with someone, anyone else.