Why is lip-lip at the head of the pack pulling the sled

From the text:

"But now Lip-lip was his dog, and he proceeded to wreak his vengeance on him by putting him at the end of the longest rope. This made Lip-lip the leader, and was apparently an honour! but in reality it took away from him all honour, and instead of being bully and master of the pack, he now found himself hated and persecuted by the pack."