The song hangs on a jazzy, minor seventh chord progression (Dm7 – Am7 – Gm7 – Fmaj7). It is sophisticated but sad. Music theorists call this the "lament bass"—a descending line that evokes a sigh of resignation. It is the harmonic equivalent of watching the sunset on the last day of vacation.
Listen closely to the breakdown at the 2:44 mark. Buried under the reverb is a field recording. You can hear the clink of a bottle cap, distant laughter, and the hiss of a cigarette being dropped into a puddle. These are not random sounds; they are the artifacts of "last summer." MOS- Last Summer