Our Gemara on Amud Aleph describes a practice of the “Tzenuin” (the modest) whereby they would dedicate a tithe, from afar, for produce from their field that is stolen by passersby who presumably rationalize this petty theft. Their goal was to save these thieves from the sin of violating eating non-tithed produce. The gemara emphasizes that this is not a requirement, but rather an extra-legal act of piety.
Sefer Daf Al Daf brings down a question from the Chikrei Lev (End of vol II, YD) as to why the Gemara calls these persons modest ones. Better titles ought ot have been “pious”, or “God fearing”, or even “Lovers of Israel”? He answers, since we have a teaching that one who takes obligations upon himself when not required is considered a fool (see Yerushalmi Berachos 2:9 and Yerusalmi Shabbos 1:2), they must conduct their behavior quietly and modestly.
This begs the question as to what the value of such modesty is. We can understand it as an effort to forestall holier-than-thou arrogance. By doing these pious acts secretly, it filters out the impulse to be noticed and receive acclaim. And this is critically important, as though all mitzvos should be done lishma (sincerely), if one is obligated, he or she should perform them regardless of the motivation. But if one is not obligated, and takes on additional practices, it must be with absolute sincerity. If my analysis is correct, this obliges a person to add a new factor in one’s calculations as to whether to take on a chumrah or not. It should not only be about whether you think you could do it, or should do it, but also if you believe you can perform it sincerely or not, without any ulterior motive. In truth, ANY time one performs a mitzvah with ulterior motives it is repulsive, yet it is ordinarily necessary because those who cannot understand the value of Torah for its own sake must perform their obligations anyhow out of fear, wish for reward or what have you, to keep order in society and basic religious function. (See Rambam Introduction to Perek Chelek where he elaborates on this theme.) Yet that “heter” only exists in regard to an outright obligation. If one is not strictly obligated, then worship out of anything other than the purest of motivations causes more harm than good.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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