on best friends

It's 2:30 AM on a frigid December night. Across the snowy grasses, you hear a single solo trombonist playing Bordogni #10, accompanied by the echo of an large empty room.

She keeps going over and over the key change into G major (at about the 1:58 mark in the video), as though she's missing something. The notes are there, the intonation is there, the tone is there...

A voice stops her, followed by the creak of an ancient wooden door. It's an older man who she's never seen before. He's carrying a guitar case. "What are you trying to do?" he asks her.

"Feel G major," she says.

He shakes his head. "You're not feeling G major; you're feeling G minor."

Baffled, she says, "Yes, the first half of the piece is in G minor."

"No, no," he says, laughing. "You're not practicing in here in the middle of the night because you want to. You're spending time with your best friend." He gestures abstractly to his guitar case. "If it's a G minor kind of night, it's a G minor kind of night. Don't force it."

He yawns, bids her goodnight, and leaves. Ever since then, that trombonist has always wondered...

...do we make our mood match the music?

...or is it the other way around?