Musar על כתובות 33:13

This is the intent of Shlomo's warning: "do not mingle with those who make changes" (Prov.24:21). If a man claims to you: "a person's mind should always be associated with his fellow men" (Ketuvot 17a), reply to him, "this refers to human beings who act like human beings. Not human beings who act like animals." Shlomo warns further: "Go from before a foolish man" (Prov. 14:7). And King David said: "Fortunate is the man that walks not in the counsel of the wicked, nor stands in the way of sinners, nor sits in the seat of scoffers" (Ps. 1:1). Our sages expounded this: "If he walks, he will eventually stand. If he stood, he will eventually sit" (Avodah Zara 18b). And "I did not sit with men of falsehood, neither did I go with hypocrites. I have hated the congregation of evil-doers, and will not sit with the wicked" (Ps. 26:4-5).

Separation in conducts consists of secluding and separating oneself from societal company in order to turn one's heart to the divine service and to proper reflection in it. This is on condition that one does not turn in this to the opposite extreme. For our sages, of blessed memory, already said: "a person's mind should always associate with others" (Ketuvot 17a). Likewise they said: "[what is the meaning of the verse] "A sword is upon the boasters (baddim) and they shall become foolish" (Yirmiyahu 50:36) - A sword is upon the enemies of the disciples of the wise, who sit separately [bad bebad] and study Torah. [What is more, they become stupid]" (Makot 10a). Rather, one should associate with the good for whatever time he needs, for his Torah study or livelihood, and then seclude himself afterwards to cling to his G-d, and to attain the ways of the just and the true service.

But there are times when the Sages permitted one to lie, for example, in order to make peace between one man and another (Yebamoth 65b). Similarly, one may praise a bride in the presence of the bridegroom and say that she is lovely and charming, even though she really is not (Kethuboth 17a). A guest (Arakin 16a) who has been well treated by the master of the house should not to say in front of many people, "How good that man is in whose house I was a guest, how much honor he paid me," lest many come to that host who are not worthy to be his guests, and concerning this it is said, "He that blesseth his friend with a loud voice, rising early in the morning, it shall be counted as a curse to him" (Prov. 27:14).