וְאַתָּה תְּצַוֶּה אֶת-בְּנֵי יִשְׂרָאֵל, וְיִקְחוּ אֵלֶיךָ שֶׁמֶן זַיִת זָךְ כָּתִית--לַמָּאוֹר לְהַעֲלֹת נֵר, תָּמִיד
And you will command the children of Israel, that they will bring you pure olive oil, pressed to be burned, to be lit, eternally.
Why first of all is the entire Jewish people being directed in the performance of the mitzvah of providing oil for the menorah, which is lit by the Kohanim.
Secondly, why do we need Shemen Zayit Zach Katit, pure olive oil of only the first drop of oil squeezed from the olive, if it's only going to be burned anyway. The answer to that is in the rest of the pasuk. Lehaalot Ner Tamid, we need only the purest oil for it to create a truly eternal light.
The Kohanim were expected to set an example in the purity of their behavior, down to the smallest of details. That is why their elevation is described as Kaper, a word usually used for atonement, because their elevation requires them to achieve a higher standard of behavior so that even their former actions which seemed accessible as Yisraelim, now require atonement in light of their new elevated responsibilities.
And now let's turn for a moment to Purim, which intersects with Parshas Tetzaveh.
The story of Purim is an uncomfortable one for modern audience, and not just because it ends with bloody fighting. But its "heroes", Mordechai and Esther seem too passive and self-effacing. By contrast, Ahasverosh, Haman are far more colorful characters.
But that too is the point. The modern sensibility celebrates egotism, and Haman and Ahasverosh were arrogant and egotistical figures, who knew what they wanted. And modern audiences are much more comfortable with them, which is what is behind postmodern revisionist reinterpretations of Vashti as a feminist heroine or of Ahasverosh as a romantic hero in the movie One Night with the King. Just as the movie, The Ten Commandments, turned the humble and self-effacing Moshe, into an arrogant posturing romantic hero in tune with the Hollywood ideal, so too modern audiences demand heroes who will exemplify egotism, rather than humility.
But egotism and arrogance taints the mixture. Everything that Haman and Ahasverosh did, they did for selfish reasons. Haman's hate emerged from a personal insult. Ahasverosh stops Haman, but only when he feels that Haman is threatening what is "his", his crown and his queen.
By contrast Esther and Mordechai repeatedly sacrifice and risk themselves. When Mordechai saves the king's life, Esther gives him the credit. Esther in turn risks her life to confront the king and expose Haman, though she would have been safe and well off in the king's palace. And when the battle is won, the Jews reject the spoils, thus leaving their victory untainted, and allowing us to celebrate it with the personal indulgence of the Purim festivities.
Esther and Mordechai were the pure drops of oil, who had to be pure in order to kindle an eternal light. The light of the Menorah was not merely a physical light, but also a spiritual one. Its power depended on the purity of its oil. This is not only the meaning of Chanukah, but also the meaning of Purim. Because the "oil" must come from the Bnai Yisrael, which is why Hashem commands Moshe to tell them to bring the oil. The fuel for the Menorah, as well as for all deeds, comes from us. And the extent to which they are pure, is the extent to which we put selfishness aside to become fuel for the light of G-d.