What was the advice Nick’s father gave him as a youth, and what has been the result of Nicks’s following this advice?

Advice....

"Whenever you feel like criticizing anyone, just remember that all the people in this world haven't had the advantages that you've had."

Result:

In consequence, I’m inclined to reserve all judgments, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores.