Playing animation in reverse plays it once only!

I'm trying to reverse the animation for my character using anim.speed =-1, the problem is that the animation gets reversed then stops. and doesn't start all over from the end to the start again. How can I make it continue playing the animation in reverse continuously. Thanks.

 public Animator anim;
     
     void Start () {
             anim = GetComponent<Animator>();
     }
     
     void myMethod() {
                     anim.speed = -1;
     }