The גמרא in שבת פח. says that at the time of מתן תורה, ד' forced כלל ישראל into accepting the Torah. The גמרא describes how ד' suspended הר סיני over the Jews"כפה עליהם הר כגיגית" and threatened them with destruction if they wouldn’t accept the תורה. The גמרא there goes on to say that in the aftermath of the events of פורים, the Jews voluntarily accepted the תורה. The גמרא derives this from the double language of the words in מגילת אסתר- “קימו וקבלו” i.e. they “ratified and accepted on themselves.” רש"י on the גמרא explains that כלל ישראל did this "מאהבת הנס".
There are two questions regarding רש"י’s explanation:
1) Why was "אהבה" specifically the factor that caused the Jews to accept the תורה?
2) To the extent that "אהבה" was what was needed, why was it דוקא the נס of פורים that engendered this "אהבה" and not any of the many previous ניסים that כלל ישראל experienced?
The מדרש תנחומא (נח אות ג)explains that although כלל ישראל accepted תורה שבכתב voluntarily with the declaration of "נעשה ונשמע", they refused to accept תורה שבעל פה until Hashem forced them "כפה עליהם הר כגיגית". The מדרש תנחומא goes on to say that the reason כלל ישראל didn’t want תורה שבעל פה was because it was too difficult to learn. The מדרש states "שאין לומד אותה אלא מי שאוהב הקב"ה בכל לבו ובכל נפשו ובכל מאדו".
So it comes out that "אהבה" was the missing ingredient that was needed to overcome כלל ישראל’s unwillingness to accept תורה שבעל פה due to the difficulty of learning it. At the time of matan torah, this level of אהבה was lacking in כלל ישראל. However on Purim, things changed. This is what רש"י means when he tells us that on פורים the component of אהבת ד' was present in sufficient strength to cause קימו וקבלו, i.e. to motivate כלל ישראל to voluntarily accept תורה שבעל פה despite the hardship involved in studying it.
Having explained why "אהבה" was the element that was needed to bring about a קבלה ברצון of תורה on פורים, we need now to answer the second question -what specifically about נס פורים generated this intense אהבת ד'?
To answer this, we need to know the יסוד of the פלא יועץ. He explains that on פורים, Hashem showed "חיבה יתירה" to כלל ישראל. How so? The גמרא in מגילה tells us that המן’s גזירה against the Jews was a result of the Jews participating in אחשורוש’s סעודה. The פלא יועץ points out that at the time of אחשורוש’s סעודה, while כלל ישראל were in the midst of sinning against ד'- so badly that they deserved to be wiped out for it- what was ד' doing? Concurrent with His anger at כלל ישראל’s חטא, ד' was manipulating events so that He could SAVE כלל ישראל from the consequence of the very חטא that they were in the middle of committing. ד' was arranging for אחשורוש to get drunk, to ask for ושתי to come in front of him and his men, haveושתי refuse to come, and for אחשורוש to then decide to kill her. All this was to enable אסתר to come in and save the Jews.
The fact that ד' was arranging to save us, at the very time that we were sinning, before תשובה and during שעת חרון אף, was a tremendous expression of אהבה that ד' has for כלל ישראל. The natural response was for כלל ישראל, upon realizing this had happened, to be filled, reciprocally, with אהבה for ד'. This surge of אהבת ד' was the missing link needed for the Jews to reach the point of "אוהב את הקב"ה בכל ליבו" (as the מדרש above referred to it) and thereby voluntarily accept תורה שבעל פה, despite its difficulty.