Musar על שבת 64:7

And a man's sons also die young because of this sin, as Chazal have stated (Shabbath 32b).

If so, why should we be "smarter" than the earlier generations? For, in truth, in this regard, we have been "without field or vineyard" more than eighteen hundred years, and, in spite of this, our G-d has not forsaken us. This [(reluctance to further the Torah education of our sons)] can only be due to the fact that the yetzer hara deceives the father into believing that he is being compassionate to his son by removing him from Torah and subjugating him to the affairs of the world. But, in truth, when he considers this well, he will see that he is being cruel to him in this and not compassionate. As we find in Sifrei on Devarim 11:19: "'And you shall teach them [(words of Torah)] to your sons to speak in them.' From this they have taught: When the child begins to speak, his father speaks the holy tongue with him and teaches him Torah. And if he does not do so, he is fit to be buried, as it is written: 'And you shall teach them to your sons to speak in them… (Ibid. 21): so that your days will be lengthened.' If you teach them to your sons your days will be lengthened, and, if not, your days will be shortened." For thus are words of Torah expounded: From the negative you infer the positive; and from the positive, the negative. (And the Sifrei speaks not only of very young children; but so long as his son is under his authority, it is incumbent upon him to teach him Torah [For does the verse specify "minors"?]) And thus have Chazal said (Shabbath 32b), that a man's sons die because of neglect of Torah. Therefore, let each man see to it to strengthen his son in Torah, and he will thereby lengthen his days — he and his sons. And the Holy One Blessed be He, who gives bread to all flesh, will certainly not withdraw His eyes from the tzaddik.

Therefore, you must know that there are none so cruel as those who commit transgressions, for through the sin of hatred for no good reason the Second Temple was destroyed and all manners of troubles come upon the world because of sins, as our Rabbis, upon them is peace, stated: "Because of the sin of failure to give Hallah there is no blessing in the harvest" (Shabbath 32b). And, "Because of the sin of failure to give Terumah and Tithes the Heavens withhold their blessing" (Ibid). And, "For the sin of twisting, falsifying and degrading the law the sword, great plunder, plague and drought come" (Ibid.: 33a), and there are many similar instances. And in truth those who cause these troubles are the really cruel ones against themselves, their children and the people of their generation, as it is said that all the people were sustained through the merit of Hanina ben Dosa (Berakoth 17b).

Let us return to my earlier statement that ההולך בלא בנים מת הוא בעולם הזה ולא יחיה בעולם הבא, "anyone who is childless is considered dead in this word and will not live in the World to Come." There is a way to overcome this handicap posthumously by means of the levirate marriage. The first thing one has to bear in mind is that every husband deposits part of his רוח, spirit, during marital relations with his wife. This is so because he has only acquired the seventh part of his צורה or דמות by union with her. The spirit he has deposited within her remains within his wife even after he has died, especially during the first twelve months after his death. All this has been explained by the Zohar on פרשת משפטים where the subject is discussed. This residual spirit of her deceased husband is at war with the second husband whose spirit is also deposited within her by their union in marriage. During the struggle between these two spirits either one may emerge victorious on different occasions.