"Therefore I speak to them in parables, because while seeing they do not see, and while hearing they do not hear, nor do they understand"
The key to the whole passage in Matthew 13:13-23 is the word understand. It appears in verse 13, verse 14, verse 15, verse 19, and verse 23 and it means to put together. However, it also carries with it a Hebraic meaning that includes “response to" or "obedience to a concept." Jesus seems to clearly be saying that if you hear but do not obey, then you are not really understanding. If you hear the message of the gospel and see the wonders of Jesus' ministry and do not respond with submission and obedience, then you will have the truth and wonder hidden from you. Because these people did not respond with full surrender to the truths but instead kept following to have their ears tickled and their bellies filled, they showed their lack of understanding. They looked like followers of Jesus, but they were just following and not really adjusting their lives to the truths and commands of the Lord.
It is not that Jesus is trying to keep people from understanding by speaking to them in parables; but because they do not respond to the word in obedience and submission but rather only want to hear something new and something pretty and something clever and interesting. They hear the stories but do not pursue the real meaning by really following Jesus. Their halfhearted obedience clouds their minds and keeps them from really understanding the depths of Jesus' teaching. They do not understand because they do not "stand under" the word for correction, direction, and training.
Notice that in verse 15 their hearts have become dull and they scarcely hear and they have closed their eyes. If they had not done this shutting down, then they would see with their eyes, they would hear with their ears (which would become obedience), and then they would really perceive or understand and be standing under the words of Jesus. He would then heal them.
This is another example in John 7:17, “If anyone is willing to do His will, he will know of the teaching, whether it is of God or whether I speak from Myself.” Real understanding requires involvement in and obedience to the word. The light only dawns completely when faith is expressed. Becoming a Christian and growing as a Christian is only possible by being obedient to the truth in front of you about Jesus Christ. There are times for mental perception and examination, but then there comes a time for action! We can never expect to completely understand God's work in the world and it will always require faith to follow where he leads.