All people have regrets. Warriors are no exceptions.
One would hope it was possible to distinguish between events caused by one's carelessness or lack of ability and those caused by circumstances or forces beyond a one's control. But in practice, there is no difference. All forms of regret sear equally into the mind and soul. All forms leave scars of equal bitterness.
And always, beneath the scar, lurks the thought and fear that there was something else that could have been done. Some action, or inaction, that would have changed things for the better. Such questions can sometimes be learned form. All to often, they merely add to the scar tissue.
A warrior must learn to set those regrets aside as best he can. Knowing full well that they will never be far away.