Statistics
I notice people are misusing statistics on this thread to reject certain of the disapparance theories as being "too unlikely".
What they should realise is that even the more straightforward explanations (e.g. decompression or pilot suicide) are themselves incredibly unlikely and have only happened a minute number of times in the entire history of air travel.
The only certainty is that whatever the explanation is, it has to be extraordinary! And this means, in my view, that you can't discount the whackjob theories, no matter how odd they seem.