The management level of Al-Qaeda are fundamentalist Islams version of a military industrial complex. They've been fighting since Afghanistan and war is now a way of life.

The foot soldiers motivation varies. Three motives that stand out for me in showing motivation to fight is complex would be:

I can't remember the source but it went that 50% of Al Qaeda caught in Iraq were Yemenis who came to fight for the money rather than anything else.

Some of the Al Qaeda just seem to do jihad as a just another thing that you're expected to do as a 'man', go to stripclubs, drink, blow up a few hundred people. Not exactly your stereotypical jihadis; examples would be the 9/11 bombers and the bluewater bombers.

Others become jihadis because how else can the evil crusaders being directed by the Jewish ZOG machine be stopped?