לֹא-תְבַעֲרוּ אֵשׁ, בְּכֹל מֹשְׁבֹתֵיכֶם, בְּיוֹם, הַשַּׁבָּת. (שְׁמוֹת לה:ג)
You shall not kindle a fire in any of your dwellings on the Shabbos day.
The question is asked by all of the commentaries: Why was the prohibition of kindling a flame singled out here, instead of any of the other 38 melachos, which are prohibited on Shabbos? The Chasam Sofer suggests the following: Imagine living in the times of the mishkan, the manna fell from Heaven—ready to be eaten. Our clothing was always fresh and clean. How would someone at that time be able to “prepare for Shabbos”? When the Torah first introduces us to the Manna, Moshe says (16:23) “Bake what you wish to bake and cook what you wish to cook (today on Friday), tomorrow…is Shabbos.” Why would the Jews have to cook or bake the Manna if it was ready to be eaten? The answer is that each Jew has an obligation to make Shabbos special by stopping all melacha, as a fulfillment of our emulating Hashem’s Shvisa. Therefore, we were given a mitzvah to cook and bake our food, though it was not necessary and by doing so, we were able to express the concept of ‘resting’ on Shabbos.
However, at this point in the desert, the Jews were involved in the building of the mishkan during the first six days of the week, and on Friday they were ceasing from all the melachos. This secession from work clearly showed the importance of Shabbos compared to the other days of the week, and so one might think that the Jews should therefore be allowed to cook and bake for the purpose of oneg Shabbos (just as it is permissible to do so on Yom Tov). The Torah therefore tells us that one who wants to honor the Shabbos should cease all work, and not think that Hashem would rather have us cooking or baking in honor of the Shabbos, on Shabbos.
The Mishneh Berurah (262:3:11) discusses the age-old problem of running down to the wire on Friday afternoon: People in the shower trying to wash their hair, or in the kitchen trying to finish decorating a cake—and then the siren blows announcing the start of Shabbos. Let’s take a step back and think about what exactly is the honor of Shabbos? Is it to spend one extra minute to finish your preparations even though it might be Shabbos already? Or, to suspend all work, even while in the midst of preparations, and even if your hair is not washed.
Bottom line: The kavod of Shabbos is delineated in a greater way by abstaining from melacha than in the preparations for the beauty of Shabbos.