Many Divrei Torah have been written presenting intricate ideas based on the concept of קיימו וקבלו in relation to Purim.
The Gemara reads:
ויתיצבו בתחתית ההר א"ר אבדימי בר חמא בר חסא מלמד שכפה הקב"ה עליהם את ההר כגיגית ואמר להם אם אתם מקבלים התורה מוטב ואם לאו שם תהא קבורתכם א"ר אחא בר יעקב מכאן מודעא רבה לאורייתא אמר רבא אעפ"כ הדור קבלוה בימי אחשורוש דכתיב (אסתר ט) קימו וקבלו היהודים קיימו מה שקיבלו כבר
"And they stood at the foot ( lit. the underside) of the mountain (Mt. Sinai) ... God held the mountain over them like a vat and said to them, "If you accept the Torah, fine; if not, your burial place will be there."... But they accepted it again [voluntarily] in the days of Achashveirosh, as it says, "The Jews established and accepted" they established in the days of Achashverosh what they had already accepted at Sinai (Shabbat 88a).
I think that this Gemara presents a rather simple truth.
At Mount Sinai, the nation had experienced ten plagues, God had freed them from their Egyptian taskmasters, they had miraculously crossed the Reed Sea, God had fed them water and Manna in the desert. He had saved them from Amalek. Now, they stand amidst the impressive spectacle of Mount Sinai. They are far from objective. They are "under the influence" of an enormous kindness and power that God had showered upon them in a concentrated period. It was "as if" God had forced them to keep the Torah. They had been brainwashed, "wow"ed beyond all reasonable logic. Who would not have wanted to accept the Torah?
The time of Purim is very different. We are in exile. The nation has been dispersed after 800 years of sovereignty; the Temple has been destroyed. Jews are now spread throughout a wide geographic expanse, from Egypt to Persia. They have good reason to wonder whether they will ever regain their honour, their land, their future as a nation! And now, they face the threat of annihilation by Haman who mobilises the entire imperial mechanism to wipe them out. Who would want to identify as a Jew? Who would want to wear a kippa in the street? WHo would want to live a life of Torah? The outlook was bleak, the dangers considerable. In Shushan, after the Hurban and the threat of Haman, whoever continued to practice Judaism, to identify as a Jew, did it only out of choice out of desire. After all, they had every excuse to quit. It was certainly unglamorous, unpopular, and downright dangerous to be a Jew.
No! קיימו וקבלו - The nation elected, against all odds, to continue to be the Jewish people!
... And that is a good reason as any, to celebrate!