Q. (See above question 3547). Why is the mitzvah of  Mishenichnas Adar Marbim Besimach not mentioned in Shulchan Aruch, while its counterpart, namely that when the month of Av begins we have to diminish happiness is actually included?
A. Ri’f (Ta’anis 9:1), Rosh (ibid 4: 32), Sheiltos 158), Magen Avraham (686: 5), Mishna Berura (ibid 8) and other Poskim do mention the above as a viable Halacha.
Chassam Soffer (O.H. 160 gives two reasons why it was not included in Shulchan Aruch and the Rambam. He explains that the increase of happiness is related mainly to the removal of prohibitions involved in building and erecting structures that create joy, which are forbidden on the month of Av. He offers another answer that deals with the Jewish nation not depending on ‘Mazal’ at all.
Nimukei Orach Chaim (ibid) explains that the Shulchan Aruch and the Rambam deal mainly with mitzvos that depend on acts and not just on feelings such as the unattached Simcha of this month. Machsheves Am (1: 1) quotes a similar answer given by Horav Chaim Kanievsky Shlit’a.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a.