We find a source for this in the Gemara (Ta’anis 10b), which brings the opinion of the Tanna that not every person who wishes to establish a fast day for himself may do so. One may wonder, why not? He’s not bothering anyone or offending anyone. Why can’t a person simply fast if he so chooses? Rashi explains the reason: Because he is seen as hubristic, and we have learned that הימנו נוחה המקום רוח הימנו נוחה הבריות שרוח כל, and having hubris would be the opposite of gladdening the הבריות רוח.
This person is doing the wrong thing by fasting! Because it brings him to hubris! You’re being different from others, and you’re deciding that you’re above them. In other words, although you’re fasting, you’re really eating—perhaps not food, but a full portion of ga’avah. The Ribbono shel Olam says, “This isn’t good for you. Stop it.”
If a person does something that’s very difficult for him without anyone knowing, there’s something to discuss—but if you’re doing something that illustrates that there are the “simple people” and you’re above them, it’s very damaging for you. And thus, behaving normally is a מצוה ...אחרים ע"י להיעשות אפשר שאי. If someone else sits and schmoozes with your relative, perhaps he will cease being a ba’al ga’avah. But you will remain one. If you behave differently from others, it causes discomfort and unpleasantness to those around you. This causes harm to them, but even more so to you. A person must be flexible not only in his outside behavior, but also in his essence.
