Judging from the lyrics, the title of the song: when love is a thing of the past.

This song was originally sung by Jonathan Lee and Sandy Lam, and was later covered by Leslie Cheung as the ending song of Farewell My Concubine.

Explanation: Don't mention the past, life has experienced many ups and downs.

Appreciation: Let bygones be bygones. After everything, it's nothing. Let's move on.