At the end of the Megillah, Mordechai sends books to all Jews, instituting Purim as an annual holiday. The Megillah tells us that קימו וקבלו “they established and received” this.[1] The Gemara interprets this as an allusion to the acceptance of the Torah:

And they stood under the mount [Sinai]:  R. Abdimi b. Hama b. Hasa said: This teaches that the Holy One, blessed be He, overturned the mountain upon them like an [inverted] cask, and said to them, 'If ye accept the Torah, 'tis well; if not, there shall be your burial.' R. Aha b. Jacob observed: This furnishes a strong protest against the Torah.  Said Raba, Yet even so, they re-accepted it in the days of Ahasuerus, for it is written, [the Jews] confirmed, and took upon them [etc.]:  [i.e.,] they confirmed what they had accepted long before.

B. Shabbat 88a, Soncino trans. e-daf.com

(שמות יט) ויתיצבו בתחתית ההר, אמר רב אבדימי בר חמא בר חסא: מלמד שכפה הקדוש ברוך הוא עליהם את ההר כגיגית, ואמר להם: אם אתם מקבלים התורה - מוטב, ואם לאו - שם תהא קבורתכם. אמר רב אחא בר יעקב: מכאן מודעא רבה לאורייתא. אמר רבא: אף על פי כן, הדור קבלוה בימי אחשורוש. דכתיב (אסתר ט) קימו וקבלו היהודים, קיימו מה שקיבלו כבר.

שבת פח.

Why, after forcibly receiving the Torah at Har Sinai, did we need to re-accept it on Purim? The Ritva, on that passage in the Gemara, explains that the Torah wasn’t accepted at Har Sinai because אין קבלה באונס; if you are forced to receive something, it doesn’t count as receiving. Commentators suggest that the revelatory experience of Har Sinai was so profound that accepting the Torah then couldn’t be considered accepting it freely. The Purim story, in which God’s involvement was hidden, stands in direct contrast to this experience. Mordechai and Esther had non-Jewish names and Jews attended the indecorous party at the king’s palace. Assimilation was rampant; choosing to accept mitzvot was an active decision. Rashi on that passage in the Gemara explains that on Purim the Jews accepted the Torah מאהבת הנס שנעשה להם, due to love of the miracle. They didn’t feel the fear they felt at Sinai and this acceptance through אהבה was stronger.

The Maharsha on that passage in the Gemara and the Torah Temima in his commentary to Esther (9:27), point out that the language of קימו וקבלו  preserves the order and difficulty of , נעשה ונשמע, we will do and then hear. How could we agree to do or establish something, before having heard or received it? In the Megillah this order is particularly difficult because we already knew what was in the Torah; the reception had already preceded the establishment. Dr. Tamar Ross has said that you cannot know that something is Divine by logical proof, but only through experience.[2] Only by first doing mitzvot, can we sense their Divinity and hear their message. Full acceptance of the Torah was more possible at Purim, when we had already experienced the observance of mitzvot. Yet even then, the order of doing and then receiving remained because we must continue to observe the Torah in order to properly to receive it.

Because God is concealed, accepting the Torah is a choice: a choice that we make based on our ahavah (love) of the experience.

[1] Esther 9:27.

[2] Class at Midreshet Lindenbaum. The first half of this dvar Torah I heard from a teacher at Midreshet Lindenbaum.