While listening to a new song, the brain is constantly predicting what's coming, the next note, when the chorus hits. After hearing a song enough times, those predictions become certain, and the brain stops encountering surprises entirely.
That predictability feels oddly comforting on a hard day, because when everything else feels uncertain, at least one thing happens exactly as expected.
That's why people having a rough day often gravitate unconsciously toward the same song. It's not about seeking something new, it's about wanting to relive something already known.