According to halacha it is sufficient to light one candle each night throughout Chanukah for the entire household. Those who are mehadrin min hamehadrin extremely careful in performing mitzvot follow Beit Hillel and add one candle each night. (Shabbat 21b)

Why, in regard to lighting the Menorah, all homes conduct themselves in the manner of mehaderin min hamehaderin supremely scrupulous while in many other mitzvot they follow lesser halachic requirements?

 

In describing the miracle of Chanukah, the Gemara relates that the Jews found only one cruse of oil, and that it had the seal of the Kohen Gadol. In the Beit Hamikdash there were Kohanim assigned to the special task of oil making. It was not the responsibility of the Kohen Gadol to make oil. Why then did this particular cruse bear the Kohen Gadol's seal?

The Kohen Gadol was required to bring a daily sacrifice consisting of flour and oil, known as "chavitei Kohen Gadol" (Vayikra 6:15). Normally, the oil used for this offering would be of lower quality than that used for the kindling of the Menorah. However, the Kohen Gadol in that time was a highly distinguished spiritual personality, and a mehader bemitzvot scrupulous in mitzvot who used pure olive oil for his daily sacrifice.

When the Hasmoneans entered the Beit Hamikdash, they did not find any oil to kindle the Menorah. Luckily they found one cruse which was for the Kohen Gadol's daily sacrifice, and to their amazement it was pure olive oil. Were it not for the fact that this Kohen Gadol was a mehader bemitzvot, no oil would have been available. To emphasize the uniqueness of the Kohen Gadol at that time, we emulate his actions in the form of mehaderin min hamehaderin.

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On the basis of the above, we find another rationale for the observance of Chanukah for eight days, though there actually was oil for the first night.

The Menorah required one half lug for each of the seven candles, adding up to a total of 3 1/2 lugim (Menachot 88a). The Kohen Gadol's daily sacrifice required only a total of three lugim of oil per day (Menachot 87b). Thus, the cruse found was not sufficient for even one night, though miraculously it burnt through the entire night.