Charles Schulz, though regarded by others as a great cartoonist, suffered from depression and feelings of inadequacy:

"It took me a long time to become a human being," he once said. "I never regarded myself as being much and I never regarded myself as being good-looking and I never had a date in high school, because I thought, who'd want to date me?"

Sensitive to slights, he never forgot the rejections of Central High. To the end of his life he remained baffled that the editors of the "Cehisean," the Central High yearbook, had rejected a batch of his drawings. At the age of 53, he made sure that a high school report card was printed in facsimile in a collection of his work "to show my own children that I was not as dumb as everyone has said I was." He sustained the traumas of his adolescence far into adulthood — far enough, in the end, to see them become a crucial element in the universal popularity of his art.

Chronic rejection and unrequited love are the twin plinths of Schulz's early life and later work. Even when he had become the one cartoonist known and loved by people around the world, he could still say, with conviction, "My whole life has been one of rejection."

...

Melancholy would dog him all his life, as would feelings of worthlessness, panic, high anxiety and frustration. It wouldn't matter that he married twice, raised five children, and became the most widely syndicated and beloved cartoonist of all time, attaining success on a scale no individual comic strip artist had ever known. Success fell off him. He was unable to take refuge in its rewards. With his first wife and five children, he moved in 1958 to a paradise among the redwoods of Northern California, where he briefly found happiness during a decade in which the work of his pen and the peaks of his professional achievements coincided with the nation's upheavals. But Schulz knew better than anyone that he could never really become a sunny citizen of the Golden State. He found little comfort in fame or prosperity or the California sun. Pain gave him his core. "I think that one of the things that afforded Sparky his greatness," a friend would say after his death, "was his unwillingness to turn his back on the pain."

Reference: Passages: The Life and Times of Charles Schulz