Why are we at war?
I would guess these reasons all come into play:
1) With fears of fundamentalist revolutions in Saudi Arabia and Pakistan, Bush wants a friendly country around and a backup place to base troops in the Middle East, both for normal geopolitical reasons (oil) and in order to try and stop terrorism there.
2) The US administration wants to demonstrate its willingness to use force when defied, in the hopes that it will have to do so less in the future. Places like North Korea, etc, are likely to be less belligerent/combative after seeing the US conquer two countries in as many years.
3) The US has been all wrapped up with Iraq for years and years, and Bush sees this as the only medium-term way out without risking Saddam or one of his sons re-arming and maybe pulling another Iran-Iraq or Kuwait.