In Walk Two Moons by Sharon Creech why does Gramps hit his best friend in the jaw?

Gramps is angry because his father is gone, and he doesn't know why. He hits his best friend because he asks about his father..... and he hits him not because of the question, but rather, because he doesn't know the answer.

Gramps said. 'One time when my father took off for six months and didn't
tell anybody where he was going, and my best friend asked me where my
father was, I hauled off and punched him in the jaw. My best friend. I
punched him right in the jaw.'