The fox rescues the little prince from the despair he had fallen into on seeing a garden full of roses on Earth—until that point, the prince had believed his flower (who was also a rose) when she told him that she was the only one of her kind in the universe.
But after the prince tames him, the fox tells him to go again to see the roses in the garden, and that this time, the prince will see that his rose is indeed unique.
And this is exactly what the prince realizes. He tells these roses:
“To be sure, an ordinary passerby would think that my rose looked just like you….But in herself alone she is more important than all the hundreds of you other roses: because it is she that I have watered;…because it is she that I have listened to when she grumbled, or boasted, or even sometimes when she said nothing. Because she is my rose.” (21.54)
(Awww.) This is the most important realization that the little prince makes about his relationship, and it is the fox’s wise words that guide him to it.