Мусар к Берешит 25:27

And every prudent man acts with knowledge, and his eyes foresee the thoughts and tricks of his children; and if he had the power, he would prevail to distance his son from “being a man that knows hunting” [(i.e. unlike Esau, see Genesis 25:27)]. And he should distance him from lies, for truth is the foundation of [all] building. And he should exert himself to distance his son from disgraceful things and from uttering oaths, and thus distance him from any trace of a prohibition, ugliness, and anything similar [Chullin 44b]. And in particular from the sins of youth one requires extra protection, knowledge, caution, and alacrity. And he should not trust his sons, nor should he be confident to say: “my son does not follow the advice of the wicked and does not stand on the path of sinners”, for there is no guardian against licentiousness (Ketubot 13b, Chullin 11b). And all who are greater than their fellows have a greater [evil] inclination than them (Sukkah 52a). And immaturity affects much, and evil companions affect much (Mishnah Sotah 7a). Therefore, one should greatly strengthen himself to distance his sons from [such] friends with all he has at hand, and to strive for them with all his might to ensure that they stay on the good road and observe the ways of the righteous (Proverbs 2:20). This is the core of the pain of raising children, and “according to the pain is the reward” (Avot 5:23). He will be happy from the product of his loins [i.e. his children] in this world, and his soul will rejoice in the world to come.

ויעקב איש תם. The fact that this trait plus the fact that he was a dweller of tents, are the only ones of Jacob's many marvelous character traits that are especially mentioned in the text teaches us to what extent a person has to endeavor not to be devious and not to engage in deceit and trickery.

To sum up: Man's sins leave an imprint not only on his own face but also tarnish the "image" of G–d. This is why the Torah phrases its request that we be "perfect, whole" with Him by using the word תמים in the plural, [when it could just as well have described this attitude as תם (sing.) similarly to the Torah's description of Jacob's character in Genesis 25,27. Ed.]