Besides who are you impressing by putting a gun to your head?
Based on another article someone pointed out, allegedly the girl was freaked out about the guns and he was trying to calm her by demonstrating they were safe. He seems to have improvised, doing something he normally would not, to try to calm her down, and forgot to check the chamber.
It's horrible and tragic, but I can understand how it might happen... possibly intoxicated, with a girl who's wigging out over the presence of guns... so
to demonstrate just how safe they are, he does something that is ordinarily very unsafe... if he can put a gun to his head and pull the trigger and nothing happens, then that proves the gun is "safe" and won't hurt anyone during normal handling... and he "thinks" the gun has been cleared, and...
Short term immediate memory for most people is between five and nine items, most commonly cited as seven. 30 seconds of stuff, or a simple distraction, can easily push memory of a clearing drill (or memory of a lack thereof) out of working memory before it's been accurately stored to more persistent memory.
If clearing the gun and dry firing it in a safe direction is not the last thing you did, and you're going to violate any more safety rules, clear the gun again. That's not foolproof either, but if that principle becomes habit, chances of a ND or tragedy are dramatically reduced.
Failure to recognize and account for the limitations of working memory can have even more serious consequences.
http://en.wikipedia.org/wiki/Northwest_Airlines_Flight_255
http://en.wikipedia.org/wiki/Delta_1141
Working Memory @wikipedia
Working memory is a very limited and fickle thing.