These two quotes are attributed to Twain:
When I was a boy of fourteen, my father was so ignorant I could hardly stand to have the old man around. But when I got to be twenty-one, I was astonished at how much the old man had learned in seven years.
It is a wise child that knows its own father, and an unusual one that unreservedly approves of him.
It is unusual for a quote to inspire me, but the first one has stayed with me for years.