I do not sympathise with terrorists; however, I think the Bush Administration's "War on Terrorism" is a joke. If Bush really wanted to get rid of terrorists, he would have authorised a covert operative cell to go in and assassinate key individuals in the terrorist organisations, not go public and declare war.
As for motivation, terrorists want to destroy America because of what they see of America. What is the general perception of America and its people? As boorish, self-centred and arrogant (see Fawlty Towers for a rather scathing example of this). Now, those of us who have experienced the world beyond our own borders know that not all Americans are like this, but when the "elected" (and I use that term very loosely) president of the country is a poster boy for everything that is wrong with the US of A, you can see why other, poorer nations would love to take the country down a couple of notches. Just a couple of thoughts, and for those of you who think that I share these opinions of Americans, you missed the point. I know better than to judge people based on a handful of examples, unlike some of you (you know who you are).