how does the description of daves death tie into what buck has learned about life ?

Though Buck certainly stands out, this trait is shared by many of the dogs in The Call of the Wild. We see this illustrated most poignantly by the death of Dave. He may be weak and in horrible pain, but to remove him from the traces is to kill him before his time. Buck's dreams of ancient times suggest that a dog's passion for work may be related to it's desire for an existence similar to that ancient existence. Dave "pleads" with his eyes to remain with the team. Dave seems to understand that when his ability to work ends, his life ends, for a life without labor is no real life. Once again the reader must be reminded of Buck's earlier existence. No matter how coddled he was, London appears to be suggesting that Buck must be happier in his present life. You can check out the GradeSaver site for more on this important theme.

http://www.gradesaver.com/call-of-the-wild/study-guide/summary-chapter-4-who-has-won-to-mastership