This is such a beautiful parsha. We have the kindling of the menora bringing physical light into the Mishkan to remind the B'nai Yisroel allowing Aharon and all of the Jews to be elevated spiritually (Bamidbar Rabbah). We learn of the great love of the Jews for the chagim such that they request an opportunity to celebrate Pesach Sheini for those who might not have been able to participate in the mitzvahs of Pesach at the proper time but who long for the chance to fulfill G-d's word. The greatness of the generations of the desert, the holiness of Moshe's trumpets, Moshe's great tefillos, even what many consider an entire separate book of the Torah (Rokeach, Shabbos Katan).
Yet many approach this week's parsha with a feeling of dread, not elevation. We see not the glories and beauties, but the horrors; the demand for meat, the rushing away from Har Sinai (Ramban, Shabbos Katan), and moreover the upcoming parshiyos recording the incidents of the Spies and the rebellion led by Korach, which have their roots in the complaints of the Airev Rav and the elevation of the Leviyim. Had not the nation believed the spies, they would not have had to remain in the desert for 40 years (Tosefai HaRosh, Yevamos, Mefarshim, Rokeach l'fi Rashi). Had Korach not looked for every opportunity to take offense at Aharon and Moshe's doings, the great rift and horrors of his rebellion and the following attacks by other nations would not have occurred.
This is a reminder that we must both look ahead and do our best to see only the good in all things and remove as many stumbling blocks as we can to prevent others from seeing things negatively. While this may seem a ridiculous burden, it can come back to benefit us ourselves in the end. At the same time, while it is patently forbidden to try to analyze why G-d might have caused certain things to occur in our lives (and certainly we shouldn't see things as punishments as everything G-d does is for the best, even though lacking Divine wisdom and complete comprehension we have no way of seeing how many things might be for our own benefit), we can look back and try to find the root causes of things which don't turn out as we wish. Over time, this can help teach and remind us that everything we do has consequences, and sometimes they are not ones we expected nor ones we would have wanted to happen.
Moshe and Aharon elevated each Levi according to G-d's command. They shaved them, anointed them, and waved them exactly as they were told. Korach behaved as he did due to his own failings and evil. The good which came from the event (the initiation of the Leviyim for all time) far outweighed the evil which followed (Korach's rebellion). Yet still we see that even from great good sometimes comes true bad, and we must learn to separate ourselves from it and even try to avoid it or prevent it if we can. Certainly in our every day doings, when we don't have direct instructions from G-d as did Moshe and Aharon, we must examine our actions thinking forward and backward, and doing all we can to ensure no harm comes from our choices.