In verse 21 we first encounter a phrase which will crop up quite a bit as the Exodus story progresses - God is going to harden Pharaoh's heart. This is puzzling: if it was God who hardened his heart, then surely Pharaoh can't be held responsible for his actions - but Exodus doesn't see it that way. It seems to be more the case that Pharaoh was already proud and resistant to persuasion, and God kept encouraging him to be a bit more of what he already was.
Why did God do that? Good question - but as the story proceeds I think it becomes clear that the answer is about demonstrating who is really in charge in Egypt. God and Pharaoh both think they are in charge, and they both keep upping the stakes. Compromise is no longer going to be a possibility. Winner is going to take all.
Personally I'm not very keen on high stakes. My natural inclination is to play safe or reach a deal before things get out of hand. But maybe that doesn't leave enough room for God's glory to be displayed.
By the way, in Hebrew culture, your heart is the place where you do your thinking, not your feeling (in case you're interested, your bowels are the place where you do your feeling, which must make for interesting Valentine's Day cards!) And the word translated harden has a breadth of meaning - for example, in Nehemiah it is the word that is often used for building or repairing the walls of Jerusalem.
So having your heart hardened is not necessarily bad - it can mean having your resolve stiffened, which can be a good thing. In fact the two words crop up together again in Isaiah 35:4 where God tells his troubled people:
Say to those with fearful hearts: be strong, do not fear!
I wonder what sort of heart God is calling us to have as a church at the moment?