Chance joined the pack at the recommendation of Reed. Mya and Reed are both distrusting to other wolves, but Chance was a trusted friend of Reed, and Reed recommended him after reassuring Mya that he is a very trusted wolf. Sure enough Chance did prove his worth to this pack and is very loyal to it.
Chance was a loner wolf, that had broken off with his previous pack do to bad relations with the alpha female, his former mate. So then after his former pack broke up, he was left to do his own thing. He was still good friends with Reed, and when Reed had his own pack, he suggested for Chance to come and join them.