A pal's last need is a thing to heed, so I swore I would not fail; (line 25)
This is kind of a sweet moment. OK, he’s dragging his friend’s corpse across the tundra, so maybe it’s little too dark to be "sweet." Still, we think it matters that he calls Sam his "pal." It shows that he’s carrying out this mission at least partly because he genuinely liked the guy. Even among tough-guy gold miners, there’s a real bond of friendship.
Well, he seemed so low that I couldn't say no; then he says with a sort of moan: (line 21)
Here’s another spot where we see the speaker’s softer side. He really feels for Sam, and does a nice thing that he doesn’t have to do. There’s a lot of stuff in here about toughing it out and keeping your word, but it’s good to remember that it all started with a kind gesture.
Now a promise made is a debt unpaid, and the trail has its own stern code. (line 33)
Here’s where friendship starts to mix with other feelings, like duty and honor. He started on this mission to do something nice for a friend, but he doesn’t talk so much about that once he’s on his way. In general, our speaker doesn’t seem like the kind of guy who would overshare about his feelings.