Why is there a mitzva to have a seudah with משתה ושמחה on Purim and not on Chanukah?
The Bach explains that by Purim we had pleasure from the feast of Achashveirosh. Therefore on Purim there was a decree to kill us and destroy our bodies which benefited by eating and drinking things which were prohibited. We fasted for three days day and night, and Hashem forgave us. Chazal enacted to have a seudah to remember and appreciate the miracle that Hashem saved our bodies, and our lives, from being destroyed.
The main decree on Chanuka was to annul avodah. This is as the Braisa brings: they tried stopping us from offering the daily tamid. Also, the Yevanim contaminated all the pure oil so we should not be able to fulfill the mitzvah of lighting neiros. The reason Hashem allowed this to be decreed against us was because we were lax in our avodah. We did teshuva by being moser nefesh for avodah. Therefore, Hashem saved us through the Kohanim who do the avodah in the Beis HaMikdash and performed a miracle with the neiros which itself is an avodah. Chazal therefore enacted to give praise and thanks which is avodah of the heart, but not to have a seudah.
The Gemara brings that there are three levels to fulfill lighting neiros: 1- one candle per house, 2-mehadrin, one ner per night for each person of the family, 3- mehadrin min ha’mehadrin, an addition ner is added each night, from one to eight. Rashi understands mehadrin to mean one who goes and searches after mitzvos. He wants to fulfill the mitzvah in a better way, having more candles and enabling the miracle to be expressed by each night’s addition. The Rabbeinu Chananel explains that mehadrin means הידור beautify; one who wants to beautify the mitzva has more candles to represent the miracle. The mitzvah of neiros Chanukah is unique in that Chazal provide three options to fulfill the mitzvah. This can be explained with the Bach’s idea: we are given the opportunity to be moser nefesh for mitzvos, to fulfill the mitzvah in a better way or to beautify them.
On each day of Chanukah we read in the Torah about the Nesi’im’s korbanos. Rashi brings that Rebi Nosson asks: “Why did the Nesi’im give first for the chanukas haMizbeiyach whereas for the contributions to build the Mishkon they gave last? Rav Nosson answers, because they said “let the tzibbur contribute and we will complete whatever is needed,” and in the end there was nothing left to be brought. The Torah writes והנשאם, with a yud missing, hinting that they were missing by being lax in their avodah. Therefore for the chanukas haMizbeiyach they came first to bring korbanos to fix their fault. We read about the Nesi’im to remind us of their love and dedication to come first for avodah, just as we find by Chanukah.
Let us all learn from Chanukah to be moser nefesh for avodas Hashem!