The sheep follow him because they know his voice.
The three things that a Good Shepherd does:
1. Shepherd knows his sheep.
Knowing Jesus means we are in a relationship with Jesus. Being a Christian is more than just knowing about Jesus. Being a Christian is knowing Jesus personally.
"I worry some of you still have not really met Jesus, one-to-one, you and Jesus alone. We may spend time in chapel - but have you seen with the eyes of your soul how He looks at you with love? do you really know the living Jesus - not from books but from being with Him in your heart? Have you heard the loving words He speaks to you?" - St. Teresa of Calcutta
We need to spend time with Him in order to be familiar with His voice. Otherwise it is easy for us to be led astray like sheep without a shepherd.
"He calls his own sheep by name and leads them out. When he as brought out all his own, he goes ahead of them, and the sheep follow him because they know his voice." John 10:3-4
2. Shepherd feeds his sheep.
Jesus wants to feed us in the table of the word and at the table of the Eucharist at mass. He also taught us the commandments so that when we obey it we will be fed, for he said "My food is to do the will of my Father." John 4:34
3. Shepherd defends His Sheep.
Jesus gives us the Truth in His word to combat the lies being presented to us in society. We are also in a spiritual battle everyday and Jesus wants to defend us from Satan who comes to steal and kill and destroy.