전도서 12:14의 Musar

Rather on all of them, he will preside over and inspect equally, judging each one and meting out punishment afterwards on each and every one according to what it is. This is what Shlomo said: "For G-d will bring every deed into judgment, [with every hidden thing, whether it be good, or whether it be evil]" (Kohelet 12:14). For just like the Holy One blessed be He does not leave any good deed unrewarded, small as it may be, so too He does not leave any bad deed unjudged and unpunished, small as it may be.

Besides all this, all the free time he has left from his affairs, if he is wise, certainly he should not waste it. But rather to immediately grasp hold of it and not be lax in it, utilizing it to toil in the affairs of his soul and the improvement of his service of G-d.

Why is this so? Because just like the essence of "watchfulness" involves putting matters to mind, so the essence of laughter is to remove from one's mind straight, rational thinking so that thoughts of fearing G-d do not enter his heart at all.