The Little Prince is full of sad things. Sure, there’s joy too, and friendship, and love, and understanding… but all of that is touched by sadness, especially once we get to the end of the book. Every time the characters come closer to understanding, they come closer to loss. Usually, it’s not until the characters say goodbye or part, that they realize their love for one another. The flower doesn’t tell the prince she loves him until they say goodbye. Similarly, even though the fox wanted to be tamed, he didn’t know the full pleasure/pain of it until the prince left him.
And the narrator is no exception. By the time the narrator writes down his story, it’s been six years since the prince left him behind. Although he can hear his friend’s laughter in the stars, that’s not the same as hearing it in person.
At the end of the book, the narrator is even unhappier than the prince, because it is harder to be the one left behind than it is to be the one who gets to move on.
Not only is every goodbye in The Little Prince a sad scene, but every meeting is too, because in each meeting the characters know that a farewell will follow.