Why did Al-Qaeda crash planes into the twin towers?

Al Qaeda attacked and destroyed the Twin Towers because they (falsely) believe that the West (i.e., the U.S. and Europe) are the aggressors in a (non-existent) war upon the people and Islamic nations of the Middle East. To them, the Twin Towers, which were home to many U.S.-based and foreign-based financial institutions, as well as numerous other corporations, symbolized the U.S., our economy, and our capitalist system. Likewise, they view the Pentagon as symbolizing the U.S. military.