He wanted to own all the fast horses himself.

And in those days every rider, especially a wild-horse hunter, loved his steed as part of himself.

If there was a difference between Bostil and any rider of the sage, it was that, as he had more horses, so he had more love.
Whenever Bostil could not get possession of a horse he coveted, either by purchase or trade, he invariably acquired a grievance toward the owner.

This happened often, for riders were loath to part with their favorites.

And he had made more than one enemy by his persistent nagging.