Rav Chaim Vital (Sha'arei Kedusha perek 1) writes: "After Adom Harishon ate from the eitz hada'as, his body and soul each became a blend of good and evil. This is the idea [referred to by Chazal] of the snake injecting 'contamination' into Chava and Adom. Through this contamination he caused sickness and ailments to their bodies and souls. This is the meaning of the pasuk, 'For on the day that you eat from it you will surely die' - death of the soul and death of the body."
Nevertheless, Chazal (Shabbos 146a) tell us that when Bnei Yisroel stood at Har Sinai this contamination ceased and they once again attained the level of Adom Harishon prior to his sin. If so, writes Rav Wolbe (Da'as Shlomo Shavuos pg. 284), we are to understand that the Torah must have penetrated all the areas of contamination i.e. the various facets of their bodies and souls. With this in mind it is clear, that without Torah, even the greatest human cannot divest himself of the evil fused in both his body and soul.
With this introduction we can understand a cryptic passage from the Chovos Ha'levavos (Sha'ar Avodas Elokim). "The Torah combines varied concepts - those mitzvos that cannot be comprehended with logic (chukim), together with the basics of those mitzvos that have a logical interpretation. This was done because the generation that received the Torah was at that time under the dominion of their desires, and their mind was too weak to comprehend even the logical mitzvos. Therefore, the Torah combined both types of mitzvos, so that one who can comprehend them will perform them as compelled by his own comprehension (a higher level than one who performs it because it is written in the Torah), while one who cannot comprehend the mitzvah on his own will at least perform it no differently than the chukim that he performs despite the lack of any logical explanation."
A superficial reading of this passage might lead one to think that the generation that accepted the Torah was on a low level and ruled by the passions and desires. He might even entertain the thought that the Torah was given specifically to such a generation and in modern times it is not needed due to our greater morals and intellectual abilities. In truth, the Chovos Ha'levavos is expressing a profound idea. The generation that accepted the Torah was the greatest generation to walk this earth. Yet, before they accepted the Torah they still contained the contamination injected by the snake and there was evil intermingled in their very being. Hence, even though they were on a high spiritual level and would have never acted on a negative trait, the very possibility that a bad trait might be aroused was enough to obscure an objective perception of the proper performance of even the logical mitzvos. Therefore, they accepted these mitzvos and performed them no differently than they performed the chukim.
Let us take for example the mitzvah of "loving one's fellow like himself." It sounds pretty simple, it's pretty logical and everyone talks about it - but how many people actually act accordingly? The Ramban comments that it is quite difficult to fulfill this mitzvah. A person wishes upon his friend everything good - as long as he himself is at least slightly better off! At Har Sinai there were those who accepted upon themselves to properly perform this mitzvah like a chok.
The Torah combined the logical mitzvos with the chukim so that we should be meticulous in their performance as we are meticulous with the performance of the chukim. On Shavuos we are obligated to accept the Torah. This includes accepting upon ourselves to perform the logical mitzvos (e.g., not stealing, damaging, or taking revenge etc.) with the same meticulousness as we perform the mitzvah that prohibits eating chazir or wearing shatnez!
Good Yom Tov!