To answer why, all you need to do is ask why the US has ever gotten involved in affairs in the area... Oil. All countries do whatever it takes to secure their 'survival'. The 'survival' of a state is made up of a number of different areas of concern. Some of these areas are energy security and economic security. The reason the US is so infatuated with the Middle East is because of all the oil (read: energy) they get from the area. In recent years, the US has been becoming more energy independent (meaning they rely less of foreign energy), but the US still relies on the oil trade in the area. To protect the oil trade, the US send in soldiers. When soldiers enter the scene, enmity and hostility breeds within the locals (due to their mistrust and stormy relationship with the US), and more people become radicalised.

It really comes down to this - we want to be there (because oil) and they don't want us to be there (because of their history with the West).