Seneca, Epistulae Morales
Among the reasons for our misfortunes is the fact that we live in imitation of others. We are not formed by reason, but led astray by custom. If only a few people did a thing, we would not wish to imitate it, but when more people start to do it, we follow suit, as if the act were more honorable because more popular. An error, when it has become common, has the status of something proper with us.
inter causas malorum nostrorum est quod vivimus ad exempla, nec ratione componimur sed consuetudine abducimur. quod si pauci facerent nollemus imitari, cum plures facere coeperunt quasi honestius sit quia frequentius, sequimur; et recti apud nos locum tenet error ubi publicus factus est.
One of the strengths of Seneca as a moral teacher is that he so often includes himself in the indictment of folly and wrongdoing, by using the first person plural (we, us, our) rather than the second person (you, your). If this passage started out "Among the reasons for your misfortunes is the fact that you live in imitation of others" and continued in the same vein, it would sound too much like preaching and carping.