'Harden Your Hearts' in the Bible

Why harden your hearts like the Egyptians and Pharaoh did? When God treated them harshly, didn't the Egyptians send the Israelites on their way?

"Do not harden your hearts as in the rebellion, in the day of testing in the wilderness.

As it says, "Oh, that today you would listen as he speaks! Do not harden your hearts as in the rebellion."

So God again ordains a certain day, "Today," speaking through David after so long a time, as in the words quoted before, "O, that today you would listen as he speaks! Do not harden your hearts."