Status Switch

The kohen enjoys an elite status even today. He is called up to the Torah first, he gives the special brachah of birchas kohanim, and enjoys other privileges as well. In the times of the Beis Hamikdash, the kohen was most certainly special as he was privileged with the holy task of the daily service.
The levi'im too were special, as they were charged with the job of singing in the Beis Hamidash, but they don't compare to their elite brethren.
Kayin was the first born son; after him came Hevel. Kaballah explains that Kayin was conceived in sin, and as such, the privilege of kehunah was taken away from the first-born and given to the kohen. It is explained further that the levi shares the same spiritual components as the first-born.
Thus, when Moshiach comes, things will change. At that time, the sin of Kayin will be purged, and so, the kohanim will be downgraded to the status of levi'im and the levi'im (representative of the first-born) will be upgraded to their rightful status of kohanim.
לקוטי תורה מהאריז"ל יחזקאל
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Timeless Wisdom

The Rishonim took great efforts to explain the reasons and the beauty behind the mitzvos Hashem gave us. In the forefront stands the Chinuch who authored a book compiling all the 613 mitzvos in a clear and concise format. The author is generally assumed to be R. Aharon Halevy, or the Re'ah, of mid thirteenth century Barcelona. In the entry he authored for every article, the Chinuch gives a brief paragraph explaining the "shorshei," or the "depth" of the mitzvah.
At times, they are quite timeless:
Regarding the mitzvah on the Leviim not to encroach on the Kohen's service and vice versa, the Chinuch writes:
"...The tasks of these two groups are precious and sacred; as such, care must be taken lest they become subject to idleness and neglect. There is no doubt that a job charged to two people or more will be subject to more neglect than a job charged to but one man alone. For many times each one will rely on the other; meanwhile, the job will not get done. This is clear to all men. Indeed, our Rabbis succinctly expressed this very point: 'The pot of two partners neither heats up nor cools down...'"
ספר החינוך מצוה שפ"ט