Hidden Gems in the Seforim of the Lubavitcher Rebbe זצ"ל
The following is based on a footnote the Rebbe wrote on a letter dated “end of Adar” 5731.
The Gemoro in Taanis (29a) states that משנכנס אדר מרבין בשמחה (when Adar begins we increase in joy). This is quoted in the Magen Avraham (686:5). The Chassam Sofer (Responsa Orach Chaim 160) asks why doesn’t the Rambam quote this (unlike the statement משנכנס אב ממעטין בשמחה (when Av begins we decrease in joy) which he does quote (in Hilchos Taanios chapter 5).
The Rebbe answers this as follows: מרבין בשמחה does not refer to שמחה של מצוה necessarily, as this is required throughout the year, whenever one fulfills a Mitzvah; rather it means a general sense of joy and happiness. The Rambam apparently is of the opinion that the Amora Rav Shmuel bar Shilas who taught this dictum (משנכנס אדר מרבין בשמחה) follows his own opinion elsewhere (Gemoro Shabbos 30b) that there is no room for שמחה other than שמחה של מצוה, and he therefore teaches משנכנס אדר מרבין בשמחה, as only then is other שמחה allowed, and that in addition to שמחה של מצוה one should increase in general שמחה as well during the month of Adar.
We can now understand why the Rambam omits this dictum in his Mishnah Torah, as he rules (Hilchos Deos 1:4) that one ought to be שמח כל ימיו and not only during the month of Adar.
This is also the reason why this isn’t mentioned in the Shulchan Aruch or in the Ram”o, as they also are of the opinion that one should always be בשמחה as the Ram”o writes at the end of Hilchos Megilah (end of Shulchan Aruch – Orach Chaim) וטוב לב משתה תמיד.