Gemara

Beraisa: "Es Mishpatai Ta'asu" refers to things that would be proper to do even if they were not written in the Torah, such as Isurim of idolatry, Arayos, murder, theft and blasphemy;

"Es Chukosai Tishmoru" refers to Mitzvos that the Satan challenges (the Yetzer ha'Ra suggests that surely Hash-m would not command such things), such as Chalitzah, Taharas Metzora, Se'ir ha'Mishtale'ach, and the Isurim of pork and Sha'atnez;

Therefore it says "Ani Hash-m" - I decreed them, you may not question them.

Berachos 33b - Mishnah: If one says, Your mercy is even upon a bird's nest...we silence him.

Sanhedrin 21b - Question: Why weren't the reasons for Mitzvos revealed?

Answer (R. Yitzchak): The reasons for two Mitzvos were revealed, and the greatest Chacham stumbled in them.

The Torah warns a king not to take too many wives, lest they veer his heart, and not to acquire too many horses, lest they cause Yisrael to return to Mitzrayim. Shlomo thought that he will marry many wives and acquire many horses and avoid the pitfalls. In the end, his wives veered his heart, and Yisrael returned to Mitzrayim to get horses.

Pesachim 119a - Question: What does it mean "Veli'Mchaseh Asik"?

Version #1 - Answer: This is one who covers things that Asik Yomin (the Ancient One, i.e. Hash-m) covers up, i.e. secrets of Torah (they may be revealed only to people with great Yir'as Shomayim).

Version #2 - Answer: This is one who reveals things that Hash-m covers up, i.e. Ta'amei Torah (reasons for Mitzvos).

Rishonim

Rambam (Hilchos Me'ilah (8:8): One should ponder the Mishpatim of the Torah to understand their reasons as much as he can. If he cannot find reasons, they should not be light in his eyes like Chulin. When Hash-m's name is put orally on wood, rocks and dirt, they become Kodesh. If one treated them like Chulin he transgressed Me'ilah and he needs Kaparah. All the more so, one may not reject the Mitzvos that Hash-m decided for us because he does not know their reason.

Even ha'Ezel: The Rambam warns against those who seek to be lenient and invent reasons for Mitzvos and say that the Mitzvos apply to only at times and in situations when the reasons apply.

Rambam (ibid.): It says "U'Shmartem Es Chukosai v'Es Kol Mishpatai va'Asisem Osam" to teach that we must guard and do Chukim just like Mishpatim. Mishpatim are Mitzvos for which the reason and benefit is clear, such as honoring parents and the Isurim of theft and murder. Chukim are Mitzvos whose reasons are not known. We may not question them. A person's Yetzer ha'Ra makes him waver about them, and Nochrim challenge them. Some examples are the Isurim of pork, meat and milk, Eglah Arufah, Parah Adumah, Se'ir ha'Mishtale'ach and all Korbanos. Therefore Chachamim taught that the world stands even on Avodah of Korbanos. Straight people merit the world to come through Chukim and Mishpatim. The Torah commanded first about Chukim - "U'Shmartem Es Chukosai v'Es Mishpatai Asher Ya'aseh ha'Adam va'Chai Bahem".

Ri Korkus: One should not say that since the reason for Chukim is so deep, he will not even try to understand them. Rather, one should investigate as far as his intellect allows. One who tries to be Metaher himself will be helped from Shomayim. What he cannot understand he should attribute to the depth of the matter, his limited intellect and his sins. There is no Mitzvah that is not exalted above human understanding. Even a simple person does not command something without a reason, billions of times all the more so Ha'Kadosh Baruch Hu would not do so! Also the Mitzvos we understand have many supreme reasons of limitless depth. "U'Shmartem Es Chukosai v'Es Mishpatai...va'Chai Bahem" - this does not refer to life in this world, for many Resha'im live long lives and many Tzadikim who observed all the Chukim and Mishpatim died young. Rather, it refers to the world to come, like Targum Onkelus says.

Ramban (Devarim 22:6): The Rambam (Moreh Nevuchim 3:48) explains the Mitzvos of Shalu'ach ha'Kan and Oso v'Es Beno to spare the mother anxiety, and even moreso, to distance us from cruelty. The opinion that we silence one who says 'Your mercy is even upon a bird's nest' holds that there is no reason for Mitzvos, they are merely Hash-m's will. We hold like the opinion that there are reasons for all Mitzvos. The reasons for only two Mitzvos were revealed, for even the greatest Chacham stumbled in them. There is a great reward for "Veli'Mchaseh Asik", one who reveals things that Hash-m covers up, i.e. reasons for Mitzvos. Shlomo understood reasons for every Mitzvah except for Parah Adumah. Things which are covered in this world will be revealed in the next. "Asisim v'Lo Azavtim" - reasons for Mitzvos were given to R. Akiva. All the Mitzvos are for our benefit, either to teach us proper Midos or to purify our Neshamos - "Va'Ytzaveinu...Kol ha'Chukim...l'Tov Lanu Kol ha'Yomim".