Depends on what version you want to hear. There are basically 4:
1. Official US version (Bush version): “a bunch of crazy guys attacked us for no reason. They’re just crazy. We never did anything to deserve this. Death to all Arabs!”
2. Official Al Quaida version: “We never did any such thing, but if we had done it, it would have been because of Israel’s aggression in the Middle East and the US support for the genocide. But we insist, it wasn’t us. It was God’s hand that moved those planes. And btw we’re freedom fighters, not terrorists”.
3. Slightly more complicated, unofficial US version (people with an IQ higher than Bush): “ok, we’ve been messing up the world for a while now, and people don’t like being bullied around like that. It was just a matter of time before something like this would happen. Perhaps we should re-think our international policy and use diplomacy more” (seems Obama also falls into that category)
4. Conspiracy theory: “It was all orchestrated by the CIA. They organised it together with Osama Bin Laden, just like they killed Kennedy”.
Even though #4 does sound ridiculous, it’s the one I personally agree with most. No, I don’t think it was exactly “planned” in that way, or at least I’m willing to believe that perhaps it wasn’t. But it was allowed to happen (perhaps certain people in key positions made things easy on purpose). And the reason was that the US needed an excuse to attack Afghanistan. Most of the hi-jackers were from Germany, so why not bomb Berlin instead? I don’t think there was a single Afghan on those planes.