While being brutal the Winter's Claw is viewed as the only one with the necessary strenght of character (and of arm) to "get the job done", Udyr needed a tribe with strong determination to serve, a tribe that was ready to undertake the weight of the greates conflict in the Freljord since the War the Three Sisters, not to mention the threat posed by the Ice Witch a.k.a. Lissandra. Hence why also Volibear pledged allegiance to Sejuani; i guess also because many sees the Avarosan as the most naive of the tribes, not necessarily as the weakest but surely not the most inclined to be prepared for a conflict, i suppose most would take this as being weak, like Sejuani believes. Also, the Avarosans doesn't seem to believe about the Ice Witch, which is only a mith for them (this is more a personal speculation based on the amount of information we have on the tribe, and the fact that we lack an Avarosan champion that knows of the Ice Witch, and i think Nunu doesn't know either, but i could be wrong on that assumption). We shouldn't also forget that Udyr isn't calm like Lee Sin and the other monks with which he studied, while he got balance over his anger it doesn't mean that was purged from him, rather he still retains it as one of his most prominent attributes imo, he just has more control over it than before after reaching balance and understanding with the four spirits he commune with.