The plane was stolen. Forget about failures that there are no reason to think happened, about explosions or mechanical failures, about suicides or searching the ocean for debris. Just figure out where a stolen 777 was taken and you'll find the plane.
Why would you steal a passenger aircraft carrying 230+ passengers and crew when you could steal a cargo-configured 777 or 747 with a crew of maybe 4? A passenger aircraft carries a lot more media attention: compare the coverage of the cargo 747 that crashed coming out of Bagram last year versus the plane that crashed recently in SFO. Plus, do you think all of these ships and planes looking for 370 would have been mobilized had the plane been a cargo aircraft? Probably not. To me, it seems more probable that this was a suicide by one of the pilots rather than a hijacking.