It is actually not very easy to know exactly what caused the 9/11 attacks. Different people have different views on this. The quotes in the previous answer are what President Bush said about the attacks soon after they happened. However, the president did not necessarily know why Al-Qaeda attacked. Let us look at some proposed reasons for the attack.
One reason is the reason given by President Bush in the quote above. This is the idea that the terrorists hate our freedoms and our way of life. In this view, they are attacking us simply because we stand for something that they do not like. This view essentially says that the attacks simply came about because other people hate our way of life and that we, as a country, did nothing to provoke the attacks.
Other people argue that the attacks were about our foreign policy, not about our values. In this view, Al-Qaeda hates America because American foreign policy is offensive to them. They believe that our foreign policy is anti-Muslim. They look to our support for Israel and our support for oppressive regimes like that of Egypt which do not allow Islamists to participate in politics. In this view, the hatred for us is based on their view that we hate them and are hurting them, not on our freedoms and our system.
We do not have any objective way of knowing which of these motives was the primary cause of the 9/11 attacks. It seems likely that both motives do have something to do with the attacks, but it is not possible to truly know which mattered more.
9/11 was a terrible terrorist attack on America. It was truly a day we all mourned together for the loved ones who lost their lifes. It was a true tradegy.
The United States has been victimized by an act of war carried out by terrorists. Evidence indicates that the attack was organized and carried out by the al-Qaida [al Queda] terrorist network. This organization is led by Osama bin Laden, who is being sheltered by the Taliban government in Afghanistan. The enemies of freedom who have attacked America will not prevail. The spirit of the American people remains unbroken, and the United States will do whatever is necessary to defend itself against further attacks and to defeat and destroy global terrorists. The fight against terrorism is not just America’s struggle, but one that calls on all civilized nations.
Although it is sad they attacked us, but why?
Americans are asking, why do they hate us? They hate what we see right here in this chamber—a democratically elected government. Their leaders are self-appointed. They hate our freedoms—our freedom of religion, our freedom of speech, our freedom to vote and assemble and disagree with each other.
They want to overthrow existing governments in many Muslim countries, such as Egypt, Saudi Arabia, and Jordan. They want to drive Israel out of the Middle East. They want to drive Christians and Jews out of vast regions of Asia and Africa.
These terrorists kill not merely to end lives, but to disrupt and end a way of life. With every atrocity, they hope that America grows fearful, retreating from the world and forsaking our friends. They stand against us, because we stand in their way.