Source: This quote is from a song “Nature Boy” (1948) written by Eden Ahbez, first recorded by American jazz singer Nat King Cole.

Complete lyrics:
There was a boy
A very strange enchanted boy
They say he wandered very far
Very far, over land and sea

A little shy and sad of eye
But very wise was he

And then one day, a magic day
He passed my way, and while we spoke
Of many things, fools and kings
This he said to me

“The greatest thing you’ll ever learn
Is just to love and be loved in return”