For me, personally who has sat at my laptop in the garden on my day off and reading copious amounts of news, things simply do not add up. Which is why speculation must be kept to a minimum.
The reasons could go from Terrorism (which someone alluded to has not been mentioned much) to the Bermuda Triangle (which hasn't been mentioned if you believe in that sort of BS.
People have mentioned about the weather at the time and any weather that can bring down an airliner will be seen on any en-route weather information system so I don't buy that as one single or most important cause.