Our Sages said, ‘A person should always anger his positive inclination against his evil inclination, and if he did not vanquish it, he should remind [the evil inclination] of the day of death’. Why did our Sages say to remind the evil inclination about the day of death and not the day after when he will have to give an accounting of his actions? It seems clear that the reason is that there are often times that a person wastes precious learning time, and the antidote is to imagine this is his last day on earth, and he is lying in bed with his energy sapping away, he then sees his life clearly and wishes he had utilized his time better to pack himself eternal ‘goods’ to take with him on his long journey. Then he will regret the time wasted and will rebuke himself sorely for being so callous, but it will be too late. So our Sages suggested imagining ourselves in that situation while we still can do something about it, while we still have time and energy to invest in packing properly and wisely, and then a person will no longer allow the time to slip by, just like he would not allow gold coins to slip through his fingers into the ocean.
ט׳ באייר ה׳תשפ״ב
"והאמת בלתי שום ספק כלל, שאם היה העולם כולו מקצה עד קצהו פנוי חס ושלום אף רגע אחד ממש מהעסק והתבוננות שלנו בתורה, כרגע היו נחרבים כל העולמות, עליונים ותחתונים, והיו לאפס ותוהו חס ושלום". (נפש החיים) 1