Umberto D (1962, Italy, dir. Vittorio De Sica) is nauseatingly sad, but it offers my favorite answer to Marcel Camus's central philosophical question:
Why not kill yourself?
[Spoiler alert, but I don't think you should watch this movie anyway, it's too painful, despite its "happy" ending.]
The old man Umberto D. has lost everything in a series of humiliating events, so at the end of the movie he decides to kill himself and his little dog, Flike, by stepping onto train tracks as a train approaches.
Flike is having none of it, however, being a terrier and therefore not worried about maintaining dignity and whatnot. He squirms out of Umberto's arms and runs into a park, followed by Umberto, who loves him.
Flike is suspicious at first, but Umberto wins his dog's forgiveness by throwing a pine cone like a ball for him.
The movie ends with Umberto playing with Flike.
And that's how a terrier answers central existential questions.