One of the things I noticed is that people really change when they realize their expiration date is coming, and they know what it is. Most of us don't: we just hope we can live as long as we can and do as much good as we can.

This quote means that some people start to live differently when they know exactly when they will die. They come to terms with it and try to make the most of their lives before their time is up. Most of us don’t know exactly when we will die, so we try to do as much as we can and hope to live for as long as possible.