The quote shown at the start of the movie, "The two most important days of your life are the day you're born and the day you find out why," is attributed to Mark Twain whereas the source of this quote is actually unknown.
Robert McCall: I've done some bad things in my life, Nicolai... Things I'm not proud of. I promised someone I love very much that I would never go back to being that person... But for you, I'll make an exception.