4 ideas
The fox explains that to 'tame' (apprivoiser) means to 'establish ties': before taming, the boy is one of a hundred thousand identical boys, but through patient, ritualized repeated contact each becomes unique in all the world to the other. The prince's rose is worth more than five thousand identical roses not because of its properties but because of the time he 'wasted' on her, and he becomes forever responsible for what he has tamed.
The fox's secret holds that one sees rightly only with the heart, because what makes a thing matter (a bond, a meaning, a hidden well in the desert) is not a visible property of the object. The pilot confirms it: the desert is beautiful because somewhere it hides a well, and a house is enchanted by a treasure buried in it that no one has found.
Grown-ups grasp reality only through quantities and credentials: tell them of a friend and they ask his father's income, not the sound of his voice; tell them of a house and they need its price before they can admire it. The opening drawing of a boa constrictor digesting an elephant, which every adult reads as a hat, shows how this literalism makes adults blind to what a child sees at once.
Before reaching Earth, the prince visits six tiny planets, each holding one adult trapped in a self-justifying role: a king who commands only what would happen anyway, a vain man who hears nothing but praise, a drunkard who drinks to forget his shame at drinking, a businessman counting stars he claims to 'own', a lamplighter obeying an obsolete order on a planet spinning once a minute, and a geographer who never leaves his desk. Only the lamplighter, serving something beyond himself, strikes the prince as not absurd, yet his planet has room for just one.