John 10:27, Jesus says, "My sheep hear my voice, and I know them, and they follow me". In Acts Chapter 13 we see God calling Paul and Barnabas to do his work. It says that they were sent out by His Holy Spirit. God calls us to serve. Often, we don't listen because we have a certain thing in mind and have laid out for Him how we want to serve Him. We will always bear the most fruit when we obey His calling on our lives, instead of telling Him what we think we want to do.