Adar
a) The wording "כשם" ("just as") implies that the main point is "משנכנס אב ממעטין בשמחה" ("When Av begins, we reduce our joy") — this is stated explicitly in the Mishnah — whereas "משנכנס אדר מרבין בשמחה" ("When Adar begins, we increase our joy") is no more than a "just as," something we learn from "When Av begins."
b) On the other hand, we see that in practice, the reduction of joy when Av begins is limited. One does not reduce in everything; there are matters of joy in Av as well31. But the increase of joy when Adar begins is without any limits, up to "עד דלא ידע" ("until he does not know").
At first glance this is not understandable. Since the whole matter of "When Adar begins, we increase" is derived from "When Av begins, we reduce," as the wording of the Gemara is, "just as," why then is the increase of joy when Adar begins greater than the reduction when Av begins, and so on?
12. The simple explanation of this: because joy must be present at all times, even in the month of Av, for there is always the command "עבדו את הוי׳ בשמחה" ("Serve G‑d with joy"). Therefore, when there comes in addition "we increase joy," it is without limit. The reduction is limited, because even then there must be certain matters of joy, especially because of the command "Serve G‑d with joy."
The inner dimension of the matter is, as was said in the maamar32, that the ultimate purpose is to carry out "נתאוה הקדוש ברוך הוא להיות לו יתברך דירה בתחתונים" ("The Holy One, blessed be He, desired to have a dwelling place in the lower realms") — that the Almighty should have a dwelling here below. And just as a person who lives in a dwelling is present in that dwelling with his entire being, so too is the meaning of "a dwelling in the lower realms": that the Almighty should be here below with all His Essence and Being, blessed be He.
Now it is written, "עוז וחדוה במקומו" ("Strength and gladness are in His place") — wherever the Almighty is found, there is joy. Consequently, in order for there to be a dwelling in the lower realms, there must be strength and gladness, for then it can be His place and His dwelling, blessed be He33.
With this we can understand why one must always be in a state of joy. Even at the times about which it is said "we reduce," one must also then be joyful to a certain measure, since the dwelling for Him, blessed be He, must be made constantly, every single moment. For this is "כל האדם" ("the whole of man"): "אני (לא) נבראתי (אלא) לשמש את קוני" ("I was (not) created (but) to serve my Maker")34. Consequently, the obligation is to be joyful constantly, every moment.
13. As for the fact that sometimes the order of avodah must be in a second line, in the line opposite to joy — this too will be understood according to what is explained in the maamar about the order in which one makes a dwelling for Him, blessed be He, by way of analogy to how one makes a dwelling for a king of flesh and blood. The first thing is that one must clean the place of every dirt, etc., and afterward one adorns it with beautiful furnishings, so that it should be fit for the king's dwelling.
So too in spiritual avodah. One must first remove the dirt, etc., and this is accomplished through the line of bitterness. And this is the avodah of the month of Av, "we reduce joy," in order to remove the matters of "ומפני חטאינו גלינו מארצנו" ("Because of our sins we were exiled from our land"). This, however, is no more than a preparation for a dwelling; the dwelling itself is made through joy.
14. As for what is stated in the Gemara regarding the matter of "When … [OCR unclear]
Notes
29) Taanis 26b.
30) Ibid. 29a, end.
31) See Shulchan Aruch, Orach Chaim, siman 551, se'if 2; the Bach on the Tur, beginning of siman 551: "אבל באכילת בשר כו׳" ("But regarding eating meat, etc."). The Magen Avraham there, sub-paragraph 1, requires further study.
32) Maamar beginning "Ki Sisa," 5716.
33) See Chagigah 5b, where some have the reading "ובבתי גואי ליכא" ("and in the inner chambers there is none") — cited in Avodas HaKodesh, part 3, ch. 57. Also according to our reading, it is explained in the Reshimos of the Tzemach Tzedek on Eichah, pp. 22–23 (Or HaTorah, Nach, vol. 2, pp. 1052–1053). This is not the place to elaborate.
34) [OCR unclear]
Key Lesson
Joy is not a seasonal mood but a constant obligation, because every moment is meant to build a dwelling for G‑d below, and G‑d dwells where there is joy. Even times of bitterness and reduced joy are only a cleansing and preparation; the dwelling itself is built through joy, which is why the increase of joy in Adar has no limit.