I don't believe that Bush himself really knows why he invaded Iraq.
There was a meeting held in the oval office about whether or not to invade Iraq. Bush, Chaney, and a few of Bush's "yes men" (advisors who never disagree with Bush, like Condoleza Rice, etc.) were there. At some point Chaney said that: "Even if there's just a 1 percent chance of the unimaginable coming due, act as if it is a certainty. It's not about our analysis, Cheney said. It's about our response."
Because George Bush is basically an idiot who thinks he's smart, he was impressed by this statement of irrational extremism. And because Bush routinely surrounds himself only with underlings who will never contradict or disagree with him, no one else in the room pointed out the foolishness of Cheney's statement, either. And thus it became known as the "Cheney Doctrine" within the Bush administration, and became the modus operendi for the Bush administration regarding iraq.
Now, if we look a little more closely at this ridiculous statement of Cheney's we would realize that it's complete nonsense. By Cheney's logic, if there is even a one percent chance that we will be invaded by space aliens from another planet, then Cheney is saying that we must prepare ourselves for such an eventuality as though it were an inevitable certainty. This is, of course, patently insane, foolish, irrational, and functionally absurd. But Bush is too stupid to cognate this, and I have to guess that somewhere in Bush's heart he WANTED to hear justification for invading Iraq, so to Bush this nonsense passed as wisdom. And no one in the room bothered to point out how ridiculous this statement was because they all were more interested in being Bush's yes men than they were interested in reason or truth.
But did Cheney know how stupid his own statement was? Good question. Perhaps he was just spouting off to hear the sound of his own self-important words, and there was no one else in the room to point out how nonsensical they were. Or perhaps he knew Bush is an idiot, and that Bush's 'advisors' would say nothing, and so Cheney was planting the justification that he knew Bush was looking for in his head. But if so, why? Why would Chaeney want to invade Iraq?
We're never going to get answers for these questions because Bush is too stupid and egotistical to even recognize that he was duped, and because Cheney is too clever to ever tell us the truth about why he wanted America to invade Iraq. The fact that Halliberton, Cheney's old company, and his cronies, have made billions of dollars from the invasion of Iraq probably has a something to do with it, though.
But in truth I suspect it all has more to do with dumb ego even than it has to do with money. Cheney has loads of money. His friends all have loads of money, too. None of them need any more money. So I think it just comes down to hubris. They did it because they could. They did it because they have that much power, and they did it because we the American people have allowed them to gain that much power. Bush is a moron. I don't think he's even conscious of his own reasoning. Maybe Bush did it because his father didn't. And he (like some adolescent) wanted to differentiate himself from his father by finishing the job his father did not finish in the first Iraq war. Or maybe Bush thinks he got a message from God, to invade Iraq, and to bring democracy and Christianity to the heathen hoards over there (Bush doesn't seem to fully recognize the difference between what he thinks is democracy and his religion). Who knows? My bet is that he doesn't know himself. But he's just an puppet, anyway. That's why the republican party ran him instead of McCain, who would have won the election handily. They knew they couldn't control McCain, and that they could easily control Bush, because Bush is an idiot and a fool.
So asking why Bush invaded Iraq is asking about the wrong person. Better to ask why the republican party, specifically men like Cheney, Wolfowitz, Rumsfeld, and Rove why they invaded Iraq. But they aren't going to answer. They're going to throw it back on Bush. That's why they wanted Bush to be their puppet-president in the first place. That's what Bush is for.