The Gemara in Megilla says that the Jews didn't want Purim to be a day on which it is forbidden to do labor. The Gemara doesn't explain why. It would be great - a day off from work!
Explains Rav Hutner ztz"l that if labor would be forbidden we would have to say havdala at the end of the day. WE DON'T WANT TO MAKE A HAVDALA [separation] BETWEEN PURIM AND THE REST OF THE YEAR!! THE SIMCHA MUST CONTINUE!!!
My sweet friend Tzvi Moshe Kantor added that for this reason we say the pasuk from Megillas Esther - "layihudim hayisa orah visimcha visason vikar" [the Jews had light, happiness, joy and glory] every havdala on Saturday night: We are ending Shabbos but Purim continues!
Love and Blessings:):):)!