Korach

The Posuk says ויקח קרח; that Korach took.
Rashi says that he took 250 great men and dressed them each in a Talis that was completely תכלת. Then they went & stood in front of Moshe & asked if a talis that's made of all תכלת needs tzitzis. Moshe said yes, and they all laughed at him, saying if one strand of תכלת is enough to exempt a whole garment made of other material, then if the garment is all תכלת shouldn't it be able to exempt itself?
What was Korach's reason for disputing Moshe? Moshe had appointed אליצפן בן עוזיאל as the נשיא, and Korach felt that he should have been appointed instead.
In reality, with his argument alone, he showed that he was not worthy of being the נשיא. The Steipler says, one who wants the נשיאות is not worthy, for he only wants the power & honor that it brings, while the one who deserves it is the one who doesn't want it. There's a story told of a great גאון who went to the חוזה מלובלין & asked him, why do all the people flock to you to learn, when I am even greater than you in knowledge of Torah, & they don't come to me? The Choze said, I often wonder the same thing, but maybe that's the reason. I wonder why they come to me, so they come, whereas you wonder why they don't, so therefore they don't.
R' Nosson Adler says, what was Korach really asking? Just like a complete garment of תכלת doesn't need tzitzis, so too with us; we don't need a leader since כל העדה כלם קדשים; we are all holy. Moshe answered that the garment does need tzitis. So too the nation, even though they're all holy, still needs a leader. The question is, if Korach feels that we're all holy & don't need a leader, then why is he vying to take Aaron's place? He just said we don't need a leader. R' Lieff says that you see from here how when it comes to your own personal gain, there's no rational thinking involved.
The posuk says,מי האיש החפץ חיים אהב ימים לראות טוב; who is the man who desires life, and loves days that he may see good? נצר לשונך מרע ושפתיך מדבר מרמה; guard your tongue from evil, and your lips from speaking deceitfully. Normally we would punctuate this posuk as, who is the man who desires life and love days that he may see good? Guard your tongue from evil. Says R' Nissim Alpert, that's not how to read it. Rather, who is the man who desires life? Love your days & only see good, then your tongue will be guarded from evil. If we only look for the good in people & situations, we will always be happy. And if we're happy with ourselves, then we won't talk bad about others. We also won't want what's not coming to us, so we won't end up like korach, who let his jealousy cloud his judgment. In his own words he contradicted himself, leading to his downfall & the deaths of thousands of people. We have to only look at the good in everything, which will in turn give us happiness & good days for the rest of our lives!