In Parshat Chukat, the Torah refers to the mitzva of the Parah Adumah (Red Heifer) as a chok, a mitzva that seemingly has no rational explanation. The Talmud cites a verse (Vayikra 18:4), “You shall follow My commandments (chukotai),” and comments: “These are the decrees of the King and there is no explanation for them… You do not have permission to think about them” (Yoma 67b). Does this really mean that there is no rationale for the mitzvot? Could it mean that we have no way to understand the mitzva’s rationale, but there is a rationale known to G-d?
If there is such a rationale, why shouldn’t it be revealed to humanity? Possibly because there were mitzvot whose reasons were revealed (specifically, that the king should not have too many wives lest they lead his heart astray, or too many horses lest he return to Egypt), and this led to the downfall of a great leader (Shlomo). On the other hand, we could argue that since reasons were given for those mitzvot, and for many others besides (such as Shabbat and tzitzit), this would seem to imply that all mitzvot do have a rationale. If the reason is not revealed, that is because it does not necessarily explain all the can be found within a given mitzva. Thus, King David proclaims, “I have seen that all things have their limit, but Your commandments are broad beyond measure” (Tehillim 119:96).
This may be at the root of the disagreement between Rabbi Shimon and Rabbi Yehudah as to whether we are permitted to seek reasons for mitzvot. Many Rishonim offer rationales for mitzvot (including the Rambam in both the Mishneh Torah and the Moreh Nevuchim). It would seem that they side with Rabbi Shimon, who permits seeking reasons for mitzvot.
According to these Rishonim, not only is it permitted, but it is a good idea to explore the rationale for the mitzvot. However, other Rishonim disagree and say that this is what our Sages warned us about when they said regarding a prayer leader (Mishnah Berachot 5:3), “Someone who says ‘Your mercy extends to a bird’s nest’ should be silenced, because he makes it seem like G-d’s ways are compassionate, when in reality they are decrees.”