You should always tell someone that you love them.
Biff's father, whom he loves dearly, is dying of cancer, and Biff realizes that this is the last time he'll ever see him.
Biff wants to say his last words to his father.
Biff tells his father that he loves him and was glad to have him for a father.
Biff tells his father about the weather and what the stock market did that day.
Biff is confident that his father knew that Biff loved him, and he feels glad knowing his father rests in peace.
Biff wonders whether his father knew that Biff loved him, and Biff feels regret every time he thinks of his dad.