For I have not shunned to declare to you the whole counsel of God.
Therefore take heed to yourselves and to all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood.
Acts 20:26-28 (NKJV)
If you bought a car, would you be satisfied if the car dealer only gave you the vehicle body without the engine or tires? Or if you were ordering food in a restaurant, would you be happy if they only brought you a part of the order? The obvious answer to both of these questions is no. It would be foolish for someone to say that they only wanted to have a part of a good thing and not all of it. We don't just want a part of something, we want to have all of it, because if we don't get all of it, we are missing something.
Paul made an interesting and enlightening comment in Acts 20:27, where Paul talked about how he had declared the whole counsel of God; he had not withheld telling the people anything. Paul did not say he just gave people Gospel highlights or he focused on his favorite topic or two, but Paul was diligent to teach people all of what God's Word has to say, and not just a part of it. It can be easy to focus on one or two things that we like, but we need the whole counsel of God, we need everything God's Word has to say.
Often people will focus on their favorite Bible topics and stay with that. Pastors, teachers, and others who share God's Word can be guilty of this in their teaching and preaching, and individual believers can have problems because they become focused on what they consider to be the most important thing in their life at the moment, and neglect other important Bible topics. God has given us so much in His Word and it is our responsibility to learn, teach, and live the whole counsel of God.