Here's one way of looking at it. A great many of the people that you meet or speak to in a given day are going to die before you do. Therefore, at some point in your life, something you say will turn out to be the last thing you say to that person. In most cases, we will never know when that is going to be.
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.
-- Og Mandino
Another way of looking at it is to ask if we would want our last words to someone or our last act toward someone to be words and acts of unkindness.