The speaker of "The Cremation of Sam McGee" shows a lot of guts in sticking to his promise. It would have been easy for him to just dump Sam’s body after he died, or at least to bury it fast and get on with his life. But he takes his promise seriously, and turns Sam’s cremation into a kind of mission. Even when the weather and hunger and exhaustion stack up against him, he keeps going and gets the job done. He’s like a sled-riding Nike ad!
Over the course of the poem, perseverance becomes a tool that helps the speaker to survive his ordeal and maintain his sanity.
The poem makes a comparison between Sam’s weakness and the speaker’s strength as a way of emphasizing the importance of perseverance.