On their fifth day together, the little prince asks whether sheep eat flowers with thorns as well. The pilot answers that sheep eat anything in their reach, and the little prince, dismayed, asks what good thorns are for if sheep will eat them anyway. The pilot, busy with his engine, doesn't answer, but the little prince keeps insisting until the pilot answers that thorns have no use at all—flowers have them just for spite.
This passage contrasts what the little prince and the pilot each find essential in the moment. The pilot is busy working with his engine, which he finds essential because it is a matter of life and death for himself—and the little prince finds his own question essential because it is a matter of life and death for his flower.
The little prince is offended by this notion and defends flowers as weak creatures who naively reassure themselves that their thorns are terrible weapons. The pilot, still busy with the engine, snaps that he is busy with "matters of consequence," which immediately infuriates the little prince. He accuses the pilot of talking just like the grownups. The little prince makes a speech about his flower, unique in the entire universe, which might be destroyed by a hapless sheep—and bursts into tears. The pilot abandons his work to comfort the little prince, assuring him that he will draw a muzzle for the sheep.
The little prince defends his flower as an innocent, helpless creature, rather than one who grows thorns out of spite. This innocence and naïveté is important to him, as is his relationship with the flower. When the pilot doesn't recognize the importance of this relationship, the little prince accuses him of being as superficial as the grownups, who cannot recognize the importance of friendship.