When faced with a possibly unpleasant future, the mind worries.
Worrying is the mind’s way of rehearsing and preparing: to avert unpleasantness.
The mind tries to solve the problem by thinking its way out, and it will continue to do so until uncertainties are realised into certainty.
Only one who has doused the fires of craving and aversion can end his worries.
But the ‘dying’ embers are easily flamed up alive again. Thus the meditator guards the sense doors of his heart at all times, until it becomes an automatic skill.
Thus nirvana may not necessarily be a permanent extinguishment, but more likely an automatic skill acquired by an accomplished meditator.