Rosh HaYeshiva, Kerem B'Yavneh
"To distinguish between the ritually impure and the pure" [Vayikra 11:47]. The Natziv writes, "Separating between the impure and the pure is a positive mitzva. Thus, if there are any doubts that can be analyzed in order to decide whether to permit something or prohibit it, the Beit Din is obligated by a positive mitzva to clarify the matter. Just as it is wrong to be lenient in a case where it is proper to be stringent... so it is forbidden to be stringent in a case where it is possible to be lenient." [Haamek Davar].
Rabbi Shlomo Wolbe, the sage of moral teaching in our generation, wrote an entire chapter about this subject in his book "Alei Shur." He writes that " frumkeit" (exaggerated stringency) is an egotistical urge which is not related at all to surrender to a higher power and that it does not lead to a closer approach to the Holy One, Blessed be He. This is because it is clear that the holy Shechina will not be revealed through selfishness, and anybody who bases his or her service of G-d on "frumkeit" is acting selfish. And even if he piles on himself many stringent actions – he will not become a pious person, and he will never reach a level of doing things for the sake of heaven.
The subject of stringency appears in the Talmud. For example, "Mar Ukva said: With respect to the following matter I can be compared to vinegar that was made from wine. When my father ate cheese he would not eat meat for the next twenty-four hours, while I do not eat meat during the same meal but I will eat it in the next meal." [Chulin 105a]. The conclusion is that a person who is not at as high a level as his father was should not be as stringent as his father was.
This issue is discussed in "Pitchei Teshuva" where the author quotes from a book named "Solet LeMincha," that one who wants to be stringent and take on a prohibition that was not accepted by the Amora'im, the rabbis of the Talmud, such as ignoring something prohibited if it is less than one-sixtieth of the total amount of food, is "like an apostate, and his loss outweighs any possible reward for this action" [Yoreh Dei'ah 116:10].
In "Chiku Mamtakim," a book published in memory of Rabbi S.Z. Auerbach, a story is told of a student who asked if he was allowed to use thin planks for Sechach in a succah, which was permitted by the rabbis of Jerusalem but which was not approved by the Chazon Ish. Rabbi Auerbach replied that it is permitted, and he added: How can you be stringent? You are only a young student, you are not allowed to be stringent using your parents' money, and you should also not cause extra expenses for your wife by being especially stringent. Rabbi Auerbach taught his students that if they wanted to be stringent they must first study the matter in depth. And they should be stringent only if they reached a conclusion that it was a halachic necessity, but they should never simply imitate somebody else. He said that the GRA was surprised to be considered pious. It is true that a pious man burns his fingernails after they are cut (Nidda 17a), but not everybody who does this (such as the GRA) is necessarily pious.
Rav Amital said that a student once asked him why he was not stringent in a certain matter about which the Mishna Berura writes that a G-d-fearing person should be wary. Rav Amital replied that it is indeed written that a G-d-fearing man should be stringent in this matter, but that it is not written that stringency will lead to a greater fear of G-d.
In a letter to the ultra-religious Badatz organization in Jerusalem, Rabbi A.Y. Kook wrote, "It is important to note how careful we must be when we try to be stringent in matters for which we could be lenient according to the law, so that we will not incur a greater loss than any possible gain."