President Johnson makes a good point on this:
Multiple can be true at the same time.
I personally think Bush was largely duped into calling for Iraq by the likes of Cheney, Rumsfeld and Wolfowitz.
Motives of Bush (the first one's a maybe but the other three are more likely): He really believed Saddam Hussein had weapons of mass destruction, He really believed Saddam Hussein was supporting Al Qaeda, He really believed he needed to bring the liberal democracy to Iraq, He wanted to complete his father's job
Motives of Cheney/Rumsfeld/Wolfowitz: He really believed he needed to bring the liberal democracy to Iraq, He wanted to complete [George HW Bush]'s job, He wanted to in/decrease the oil price (could be either, I don't really understand how the oil industry works if I'm being totally honest), He wanted to increase business opportunities for American oil/building companies, He wanted to increase American military presence near Russia and China