This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Little Prince
The book is entirely a tearjerker itself.
"I shall not leave you."
The end when the little prince returns to his asteroid.
Or, if you interpret it differently, the prince returns post-mortem.
And on a meta level, interpreting the narrator as Saint-Exupéry himself, who died in World War II soon after writing this novel. Then read the epilogue where the narrator asks for word if the prince ever returns...
Not to mention when the prince finally realizes that he must return to his rose, and consequently must abandon his now-tame Fox. It's worse if you've seen the movie, where the Fox is played by the gentle Gene Wilder.
"Ah," said the Fox, "I shall cry."
And then of course the Fox's last lesson for the prince, especially if you think of its importance near the end of the book... say with the laughing stars:
And now here is my secret, a very simple secret: It is only with the heart that one can see rightly; what is essential is invisible to the eye.
The exchange between the narrator and the Prince in chapter 6.
"One day," you said to me, "I saw the sunset forty-four times!" And a little later you added: "You know — one loves the sunset, when one is so sad..."
"Were you so sad, then?" I asked, "on the day of the forty-four sunsets?" But the little prince made no reply.
"I stopped in my tracks, my heart torn asunder; but still I did not understand."
A meta one, but still very tragic nonetheless: the author's plane was shot down by a German pilot during WWII. When the pilot learned many years later that he killed the very person who wrote the book he and his children loved, he broke down in tears.