What does London mean when he writes about the “paradox of living”? What are his examples of this?

The "paradox of living" has to do with Buck's perception of life.... really living. Despite the enormous hardships of living in the far north, Buck feels more alive than he ever felt in his pampered life in California. It is a paradox that pain and struggle for survival could be a cathartic moment in Buck's life. Although Buck was beaten close to death, he was able to define himself as the alpha dog of the pack: he belonged in the wilderness of the North.