"Beginning today, treat everyone you meet as if they were going to be dead by midnight. Extend to them all the care, kindness and understanding you can muster, and do it with no thought of any reward. Your life will never be the same again"

This quote is encouraging the reader or listener to live their life with compassion and generosity towards everybody. The idea of dealing with everyone as if they were going to be dead by midnight implies that every interaction with somebody ought to be treated as if it is the last, and therefore ought to be as favorable and caring as possible. The author recommends that doing this might alter the method the reader perceives the world and create a more positive, fulfilling life. The idea is to extend goodness to others without expecting anything in return, which can result in a more meaningful presence.