This song is about always being optimistic, even if your life is awful.
Eric Idle sings this at the end of Life Of Brian. Idle plays the lead crucifee.
This song was remade in 1997 by Art Garfunkel for the Movie As Good As It Gets starring Jack Nicholson and Helen Hunt. The lyric "Life's a Piece of S–t" is replaced as "Life's a Counterfeit." It can be heard at the end credits