Hello. You forgot to mention that this question is about the book "The Call of the Wild". It is necessary that you always provide all the necessary information for your question to be answered efficiently.
Answer:
Because he loves to work pulling the sled and is sick.
Step-by-step explanation:
Dave is a hard-working dog who is happy to do his job of pulling the sled. he feels fulfilled in being able to accomplish his task.
Dave got very sick and still didn't want to leave his obligations behind. The drivers felt sorry for Dave, but they believed that taking him away from his job would make him sad, make his illness worse and make him die unhappy, for that reason, they think it is an act of mercy, let Dave die doing what he loves, pulling the sled.