Imagine a pastor who taught every Sunday, just telling stories or using parables, and he never explained. You can imagine what people would say - the grumbling, the complaining. "Why didn't he explain? What are we paying this guy for?" People would leave that church scratching their heads. "I didn't get nothing out of that."
How about you? Now think back about Jesus. People walked long distances to listen to Him on the hillsides as He taught, and Jesus used parables and symbolism in His teaching. Sure, many went away scratching their heads, feeling they had wasted their time coming to see the great teacher.
Jesus privately explained to His disciples why He used parables.
Luke 8:10 (New King James Version): "And He said, 'To you it has been given to know the mysteries of the kingdom of God, but to the rest it is given in parables, that seeing they may not see, and hearing they may not understand.'"
Jesus was teaching them two things. First, the human mind cannot understand spiritual things. Paul explains in 1 Corinthians 2:14: "But a natural person does not accept the things of the Spirit of God, for they are foolishness to him, and he cannot understand them because they are spiritually discerned."
No one understands the Bible with his mind. The Holy Spirit is given to us and lives in us to lead and guide us into spiritual truth. Our spirit informs our mind. That's the only way a man can understand God or the Bible - when the Holy Spirit reveals it to us. John 16:13, Jesus is speaking here: "But when He, the Spirit of Truth, comes, He will guide you into all the truth. He will not speak on His own; He will speak only what He hears, and He will tell you what is yet to come."
Second, God gives us the ability to understand when we're willing to do what He says. Bill Johnson explains Jesus' reason for using parables in Luke 8:10. Bill said, "Jesus hides truth in parables so those who are not ready to obey will not understand, because if He increases the knowledge of those who are not ready to obey, He just increased their accountability. It's the mercy of God for Him to conceal truth and then make it where only the hungry can find it."
James 4:17 says, "If anyone, then, knows the good they ought to do and doesn't do it, it is sin for them."
Jesus revealed the parables to His disciples because they were ready to understand and obey - were teachable. Those in the crowds who were seeking truth and ready to obey would understand. That's true for us. The Holy Spirit reveals things to us only when we're ready to change or to actually do those things. Otherwise, we would be in sin, not doing what we knew we should. The measure of your desire for God, your pursuit of God, and obedience to God will determine how much God reveals spiritual truth to you.