I agree with DeadCentre - don't speculate - go back to what El Quaida says. In this context I was surprised to see that no mention had been made in this thread of Osama's videotape of October 2004 in answering this question. So I dug around a bit and came up with the fact that there had been various translations. The following pretty much sums up this situation:
http://www.fortunecity.com/meltingpo...amadisinfo.htm

It seems that the major US news networks left out much of the start of Osama's message. For me it was yet another example of the critical role of the US media in the International history (particularly) of the past decade, and helps to answer why many in the US think so differently to much of the rest of the world.

In particular the bit that had Europe all agog: Osama - You American people -- can you ask yourselves why [there is] all this hate against America and Israel.

And he immediately goes on to answer the question:
The answer is clear and very simple, that America has committed so many crimes against the nations of Muslims. America is the head of criminals by creating Israel -- this continuous crime for 50 years.
The government which is besieging the people of Iraq and killing them.

I presume in his first admittal of his role in 9/11 he would get his reasons right. But this meassage was witheld from the bulk of the US population by media editing.

Still, in my view the views of less radical people should rather be heard. I therefore find the following a much more more interesting question:

"Why have so many people who were formally friends and supporters of the US, turned against it in such huge numbers?"

The answer to that might be taken more seriously by the average "American" than the rantings of some bearded ego-head.