This is the fruit of denying the absolutes of God's moral law. It is contrary to the facts. People inevitably suffer in this world and especially in the world to come for disobeying God's law. Loving parents know this and wish desperately to save their children from the ruinous consequences of sin. No doubt His aim, in part, is to emphasize the awful wickedness of sin.
Sin destroys lives and consigns people to eternal misery.
We all need large views of the badness of sin. Another reason would be to demonstrate His authority as God. He has the right to decide how sin should be rebuked. We must honor that right. But perhaps the ultimate reason that God has appointed the rod has to do with the connection that He has created between pain and conscience.