Although Jesus spoke to many people, they did not hear Him or see what He was trying to show them. This was no new thing for Israel. Jesus was quoting almost directly from Isaiah 6:9-10 - which explained why God sent His people into exile over 500 years earlier - they had refused to listen to Him. Most of the kings had been apostate and they (and the people) ignored prophet after prophet. Their hearts were hard.
 
Jesus applied the same prophecy to the crowds who followed Him and eventually demanded His crucifixion. Their hearts were hard. It was not a sudden event, they had become practiced in hardness to the point at which they could not feel, see or hear the truth.
 
There is another edge to Jesus' quotation: the thought that because the religious majority had decided to avoid the truth, God had confirmed their hardness. 2 Thessalonians 2:10b-12 says, "They perish because they refused to love the truth and so be saved. For this reason God sends them a powerful delusion so that they will believe the lie and so that all will be condemned who have not believed the truth but have delighted in wickedness."
 
It is a solemn tragedy to continue in hardness of heart against God, His Son and His truth. There comes a point at which God allows people to have their way and confirms their hardness, blindness and deafness. Even the glorious gospel means nothing to them. Never get to that point, and warn your friends to ... "Seek the Lord while He may be found; call on Him while He is near." (Isaiah 55:6). Practiced rebellion is eternally dangerous.