This was the pride of Dave as wheel-dog, of Sol-leks as he pulled with all his strength; the pride that laid hold of them at break of camp, transforming them from sour and sullen brutes into straining, eager, ambitious creatures; the pride that spurred them on all day and dropped them at pitch of camp at night, letting them fall back into gloomy unrest and uncontent.

This was the pride that bore up Spitz and made him thrash the sled-dogs who blundered and shirked in the traces or hid away at harness-up time in the morning.
Likewise it was this pride that made him fear Buck as a possible lead-dog.

And this was Buck's pride, too.
He openly threatened the other's leadership.

He came between him and the shirks he should have punished.

And he did it deliberately.