Al-Qaeda: Why they fight?

On September the eleventh the world was forever changed. Four commercial jet liners were hijacked by Al-Qaeda members who were trained in Afghanistan, two of those planes were intentionally slammed into the world trade centers, the third was slammed into the Pentagon, and the fourth was brought down over Pennsylvania. The magnitude and scope of the attacks shocked the world, never had the world witnessed such an act of terrorism, and never had the world witnessed an attack in which commercial jet liners were used as actual missiles. In the aftermath of the attacks many people were asking who could do such a thing and more importantly why would someone commit such a heinous act? What would actually drive a person to crash a plane into a building? What possible reason could there be for such a devastating attack?