Scripture does not say that people who are not sheep cannot become sheep. That is simply your opinion. God made us all. They listen to their owner's voice. Now I reject you.
All people are by nature children of wrath.
They have to become sheep. It is how they become sheep that is causing the disagreement. You suppose that people become sheep by regeneration. The context says nothing about regeneration. Rather it speaks about listening to Christ and looking to Him as a shepherd.