The Farmer Who Was Too Eager
A wealthy prince owned large tracts of land, and made the local villagers farm them. According to their abilities, the prince gave each farmer a plot of land to plow, plant, reap, and so on. The entire harvest went to the prince.
One day, one of the villagers approached the prince with a complaint. "Why is it that you only gave me one acre to farm, while you gave one of my neighbors two acres, and to another neighbor you gave three?!"
When the prince heard the farmer's words, he immediately ordered that his plot of land be taken away from him.
The villager was shocked. "Not only did you not give me more land, but you even took away what I had before! Why?"
The prince replied, "From your words it is clear that you are dishonest. You only want more land so that you can steal even more produce from me. Why else would you volunteer to do more work by being responsible for additional land?"
You Have Enough!
In the same fashion, Moses told Korach: If you were sincere in your service of God, you would not be requesting the service of the Kohanim too. "רב לכם בני לוי" - Is it not enough that God gave you the special service of the Levites?
If your intentions were pure, just to serve God, then you already have enough Divine service. You should focus on fully performing that avodat Hashem.
The fact that you seek additional service indicates that you have ulterior motives. You are not seeking to serve God; you just want the special honors and privileges of being a Kohen!
(Adapted from Mishlei Yaakov, pp. 348-349)