In the Underworld, there is an unwritten rule that says that you will be crushed whenever you let your guard down, even if your carelessness is only as big as a grain of sand, because your opponent will definitely be aiming for that crucial moment.
Perhaps, David has been at the top for too long, all this time he is revered as the most feared and respected figure in the Underworld, so he forgets the struggle he did to reach the top.
But David's stubborn nature made Henry give up on reminding him. All this time, the thing that can make David aware is his own experience when he went directly to the battlefield.
Henry only hoped that David would not be left with the lack of alertness he was facing now. Because the longer he let his guard down, the more accustomed David would be to it. What Henry feared was that David would no longer be able to tell when he was off guard or not.
