Because of the color of the wheat

"Come and play with me," proposed the little prince."I am so unhappy."

"I cannot play with you," the fox said. "I am not tamed."

“…What does that mean-- 'tame'?"

"It is an act too often neglected," said the fox. It means to establish ties."…"To me, you are still nothing more than a little boy who is just like a hundred thousand other little boys. And I have no need of you. And you, on your part, have no need of me. To you, I am nothing more than a fox like a hundred thousand other foxes. But if you tame me, then we shall need each other. To me, you will be unique in all the world. To you, I shall be unique in all the world...

"… if you tame me, it will be as if the sun came to shine on my life. I shall know the sound of a step that will be different from all the others. Other steps send me hurrying back underneath the ground. Yours will call me, like music, out of my burrow. And then look: you see the grain-fields down yonder? I do not eat bread. Wheat is of no use to me. The wheat fields have nothing to say to me. And that is sad. But you have hair that is the colour of gold. Think how wonderful that will be when you have tamed me! The grain, which is also golden, will bring me back the thought of you. And I shall love to listen to the wind in the wheat.."

"What must I do, to tame you?" asked the little prince.

"You must be very patient," replied the fox. "First you will sit down at a little distance from me-- like that-- in the grass. I shall look at you out of the corner of my eye, and you will say nothing. Words are the source of misunderstandings. But you will sit a little closer to me, every day..."

The next day the little prince came back.

"It would have been better to come back at the same hour," said the fox. "If, for example, you come at four o'clock in the afternoon, then at three o'clock I shall begin to be happy. I shall feel happier and happier as the hour advances. At four o'clock, I shall already be worrying and jumping about. I shall show you how happy I am! But if you come at just any time, I shall never know at what hour my heart is to be ready to greet you... One must observe the proper rites..."

So the little prince tamed the fox.

And when the hour of departure drew near--

 "Ah," said the fox, "I shall cry."

"It is your own fault," said the little prince. "I never wished you any sort of harm; but you wanted me to tame you..."

"Yes, that is so," said the fox.

"But now you are going to cry!" said the little prince.

"Yes, that is so," said the fox.

"Then it has done you no good at all!"

"It has done me good," said the fox, "because of the color of the wheat fields."

From The Little Prince (1943) by Antoine de Saint-Exupery