Always tell the people in your life how much you love them and care about them. Even if you are upset with them. Just let go of your anger or sadness at them; it is not worth it. Don't let them leave upset with you without telling them that you care about them. Let them know that you love them, that they matter.
You never know when will be the last time that you ever see them. You never know when you might not get another chance.
So treat each moment like it might be the last. Because one of these times, it will be.