Life at best is bittersweet

"Life at best is bittersweet" is a recurring ethos in some versions of Teenage Mutant Ninja Turtles. It originated in and is referenced most in the Mirage continuity, but also appeared in the 2003 TV series. The phrase implies that life has its joys, but also many sorrows, and that even the best-lived overall life experience includes both good and bad memories, averaging out as a bittersweet experience. True to the phrase's assessment, the lives of most core characters in the TMNT franchise tend to be very complicated and laden with personal baggage, as they struggle either with living as mutants on the fringes of society, or as humans whose lives are deeply embedded in the secret world of mutant friends they can't talk about, or as people in general facing lives of frequent hardship and violent conflict.

Mirage
The phrase first appears in Kirby and the Warp Crystal.

It is the last message ever sees from his new friend, who had just become permanently stranded in another world when a portal back to  separated them. Don read the message with the knowledge they would never see each other again, and he might never know Kirby's ultimate fate.

The ethos is echoed by a 45-year-old in the story Loops, Part 1 of 2, when conversing with.

At the time of this story, the 15-year-old Leo had been trained in ninjutsu, but had not yet faced the for the first time, and was still relatively innocent and inexperienced. By the time he would be 45, he had already known extreme suffering, loss, grief and regret from a life filled with violence.

2003 TV series
The phrase appears in the episode The King, which is an animated adaptation of Kirby and the Warp Crystal.