Sinatra sings Sondheim:   "  We had a good thing going"   was written for " Merrily We Roll Along" a relatively unsuccessful Sondheim musical ( with a small role played by Jason Alexander aka George Costanza from Seinfeld.)

Sondheim is a master storyteller in song. Sinatra's greatest gift, among other things, is his ability to portray the dramatic elements of the music and especially the words-the songwriter's story intent.
Unlike other songs we have been featuring about the glad/sad aspects of romance, this song honestly admits that there were good things about the relationship but which is now gone. Bob Hope used " Thanks for The Memory" as his theme song. It also acknowledges the joy that once was but has now passed.

https://youtu.be/C3z-cz-bURM