There are many reasons I think. In essence, why do you invest in one stock over another....why do you invest in anything at all? A better or at least some ROI; right?
I think his personal motivations were because he and his group are dubious of our influence not only in the Middle East but the world in general. I think most people are like that at least on some level. Look at this and other forums for example and you see the hatred of Israel. They don't hate Israel because of their matsaball soup; its because a great many Hebrews and Jews have ascended to positions of wealth and influence in the West.
The big difference is that people in this country would never ever think about turning this bitter resentment into an armed conflict.
OBL has no such apprehensions and the means to make it come true except its not a (strictly) Israel/Jew equation.
I think on a grander scale, his Jihad only gains when we act aggressively. Or at least that is what the old arithmetic says; not so much since 2006 or so.
Boots on the ground in Iraq = X number of Jihadists.
Boots on the ground in Afghanistan = Y number of Jihadists.
Boots on the ground in Iran = (likely) X+20% number of Jihadists.
Boots on the groun in Saudi= X+Y+50% number of Jihadists.
OBL's problem is that he's not the only game in town any longer but he's the best funded. In 2011 and beyond; look for some tribal conficlts to emerge (if not sooner) as in-fighting takes hold.
But back to your question; OBL wanted to see American boots on the ground in Saudi most of all. It didn't work (yet). More Jihadists means more visibility for Al Queda and, of course, himself on the world stage.
A third and probably overlooked reason is simple arrogance that goes beyond hating our influence and wanting to see the big kid on the block get beat; I think it goes to the man himself. Islam, though a peaceful religion that I don't fully understand--hell I don't understand all of Christianity yet

--is very restrictive and from what I can tell, is one of those that has a culture of "you're with us or against us" with very little room for lipservice from the faithful. I think that OBL looks at the map and sees shades of gray all over it now. There are few hardline countries out there anywhere. Iran is thawing with internal strife; Hezbollah is funding schools; while Israel-Arab relations are not wonderful, they are not bombing one another's capitol cities, India and Pakistan are not close but are not on the brink of nuclear anhilation any longer.... I think as probably one of the last hard-core hardliners out there, OBL is seeing his worldview slip away and frankly is wanting to hit while he still can and force us back into camps; the East and the West; Christians and Jews and Muslims; Old World and New World....
Think of the first time you took the carving knife out of your dad's hand on Thanksgiving and telling him that you'd cut the turkey; was that comfortable for him? Likely not. The world is moving into a large melting pot and it is growing more uncomfortable everyday for those who do not accept it readily. I'm one of those at times.
Anyway, thats my two cents. Commence firing.