Our Gemara on Amud Aleph notes that Rabban Gamliel and Rabbi Yochanan Ben Zakkai conducted themselves with extra legal stringency in not eating anything out of the Succah. Yet notably the Gemara reports that Rav Zaddok was lenient in this as well as a variety of other matters related to eating.

There are a number of interesting, and possibly related facts:

• These Tannaim were from the generation of the Churban, of which it is said in Bava Metzia (30b):

Rabbi Yoḥanan says: Jerusalem was destroyed only for the fact that...they established their rulings on the basis of Torah law and did not go beyond the letter of the law.

• Another interesting fact is that Rav Tzaddok was not an “easy going guy''. While for whatever reason he was not machmir here, the Gemara Gittin (56a) tells us the following about him:

Rabbi Tzadok observed fasts for forty years, praying that Jerusalem would not be destroyed. He became so emaciated from fasting that when he would eat something it was visible from the outside of his body. And when he would eat after a fast they would bring him figs and he would suck out their liquid and cast the rest away

How do we square away this maximum abstention with Rav Tzaddok being extremely lenient here, unlike his colleagues?

To explain this, allow me a bit of a derush and poetic license. Tosafos on Bava Metzi’a (30b) notes that this was not the only reason offered for the destruction of the Temple, as the Gemara in Yoma (9b) says it was due to baseless hatred.  Tosafos answers that, actually, both baseless hatred and not going beyond the letter of the law led to the Temple’s destruction. 

These two reasons are not just a combination but related. Sometimes your chumros (extra-legal stringencies) can lead you to irritability and sinas chinam (baseless hatred). In those cases, your chumros are not helpful. Other times, indulgence by trying to get away with whatever you can legally can be hedonistic and selfish. This too is false and can lead you to sinas chinam because it comes from narcissism and an unwillingness to step out of yourself.  This is when chumros and voluntary abstentions are a good way to reduce self-centeredness.

Thus, Rav Zaddok, Rabban Gamliel and Rav Yochanan Ben Zakkai’s generation struggled with the appropriate balance of chumros to strengthen character and discipline versus chumros that make one bitter and angry. Great Sages such as them succeeded in that balance, which was why Rav Tzaddok had no problem eating out of the Succah on that day and utilizing many other leniencies. While this was out of character for him, perhaps he simply felt his patience thin that day and feared he might lose his temper. Of course it goes without saying, but I’m going to say it anyway, that such a holy person such as Rav Zaddok who fasted for 40 years certainly had plenty of self-control and equanimity. That still does not preclude him from having a day where he needed to know that he needed to take it easy on himself, on his level. I think that’s the exact point.

There is a psychological and rabbinic concept that if you are not mindful of your boundaries, and behave in a disproportionate way, in the end your lack of balanced priorities can cause you to stumble. Even in a manner that could be thought of as kind or righteous, if done at the wrong time, leads to trouble. For example, we find a Midrash Tanchuma Metzora (1): R. Eleazar said, “Anyone who becomes merciful upon the cruel, will end up being cruel to the merciful: It is written (I Samuel 15:9), ‘But Saul had pity upon Agag and upon the best of the sheep and the cattle’; and it is [also] written (I Samuel 22:19) about Nob the city of priest, ‘And he smote Nob the city of priests with the edge of the sword.’“

Thus, even the saintly Rav Tzaddok, who barely ate at all, knew when he shouldn’t be too strict on himself, and on that particular day, ate outside of the Succah. I believe that Rav Tzaddok used the following criteria: If the stringency could affect his mood and relationship with others in a negative manner, then he chose not to abstain.