Continuing last week's discussion of ta’amei hamitzvos, see the Iggra d’Kallah (available online http://www.munkatcherseforim.com/) end of Parshas Naso. Very roughly translated:
“The philosophers ask how it is possible through a physical mitzvah to receive [spiritual] reward…They therefore postulated that the reason for mitzvos is that man should not fall prey to his base physical nature, and they invented reasons that apply to each mitzvah. According to their reasoning, if one were to contemplate these rational ideas, one would not need to perform the actual physical mitzvah act… This is why the Torah does not reveal the reason for mitzvos. The action of the mitzvah performance itself draws one close to G-d…and one is obligated to perform the mitzvah without considering its reasons. While there surely is a reason for each mitzvah, and one is surely obligated to consider why a mitzvah was commanded, one can never reach the ultimate reason for a mitzvah, [because a mitzvah] reflects G-d’s wisdom, and G-d and his wisdom are infinite.”
I am not sure that the Rambam would agree that the performance of the physical mitzvah act is an end in itself and not a means to mental and spiritual growth (see end of Hakdamah to Peirush haMishna). However, I do not think anyone would dispute the point made that rational explanations for mitzvos do not supplant Hashem's tzivuy as the motive for their performance.