In order to save Nymeria from Joffrey, Arya Stark throws a stone at her and makes her leave. For quite a long time, Nymeria's status remains unknown. However, in the S7E2 Nymeria and Arya have a "reunion". Nymeria has become an Alpha (leader) wolf of a pack in the Riverlands.
When they meet again, Nymeria "orders" the rest of the wolves to not attack her. This action shows that the bond and affection between Nymeria and Arya are still strong. Many fans would like for Arya and Nymeria to reconciliate -basically for Nymeria to abandon her pack and follow Arya-, however, it is important to understand that Nymeria has changed. She still loves Arya, but has changed and won't just go back to following her. What most of you probably never thought of is that she would most probably have children of her own now (considering her age) and ties to other wolves which all would be too much to give away in order to return to Arya.