Hashem made unbelievable ways for us to know when we're wrong. Maybe Hashem Himself could send us signs. Maybe other people could tell us that we were wrong. Maybe something we do fails, which is an indication that we were wrong. There are a lot of ways to know that we're wrong.
Hashem said what He's going to do to Paro because Paro is living such a lie. As Hashem says, “I'm going to allow you to keep living the lie.” The plagues that should be an inspiration and a clear sign that you're wrong are not going to inspire you. It doesn't mean that his heart literally turned into iron. It just means that the plagues that should be there to show you the truth aren't going to work for you. They're not going to show you the truth. “Free will—you’re still going to have free will,” says the Da'at Sofer. You still have a choice. Moshe Rabbeinu goes into you because you still have a choice. But the hardened heart just means that if you live such a life of a lie, Hashem says, “I'm going to allow you to keep living the lie.” And the plagues, these things that should inspire you, are not going to inspire you because you are so against seeing the truth.
This is a decision every person who has stubbornness has to make. Are you emotionally stubborn, which is an incredibly positive quality? Or are you mentally stubborn, which is a very, very dangerous quality? Are you the kind of person who can blaze through challenges, take punches, and just keep on going, which is literally the hallmark of the greatness of our nation? I'm kesheh oref (stubborn) in the most positive way possible. Or are you the kind of person who lives a lie? Are you the kind of person who is kesheh oref in the most negative way possible? Someone tells you the truth, and you don't hear it. You see the truth, and you don't hear it. You watch failure, and you don't hear it. People are yelling at you, and you don't hear it. It's messing up. You're taking substances; you're messing up your marriage, your job, your income, your religion, and your children. It is so obvious that it's not working, but you are like Paro. You're so convinced to live a lie, you're so convinced to live the way you've been living, and you're so mentally stubborn that you're literally kesheh oref: when someone talks to you, you can't hear it. There's nothing more negative than that, because you can never get out of your problem. You have a problem, and there's no way to fix it, because you don't hear anybody. You don't understand anybody. You're never open to see the truth.