What did Doc Wilson mean when he said, “Nothing in the world ever comes quite free”?

This is a harsh fact that this world gives us nothing in free. To get or achieve something we need to pay an amount for the same or if we are given something, we need to repay for it. According to Doc Wilson as Penny’s life was saved by the life of doe, so it is their responsibility to repay i.e. bringing the fawn and raise it.