It states (Divrei HaYamim 1, 16:27), במקומו וחדוה עוז, "Might and joy are in His place." Hashem resides in a place of joy. The Chasam Sofer zt'l (Drashos 7th of Av, 5560, p.307) writes, "Hashem resides with joy, and Chazal say that Hashem doesn't reside in places where there is sadness. Nevertheless, Tisha b'Av is called מר יום, a bitter day. Therefore, it is proper for all the bitter and broken people to cry and mourn on this day when Hashem is also mourning. If they do so, their mourning will go up to a very high place. Although they don’t see it, their mazal sees, and they will hear the voice of Hashem crying together with them... Chazal say, 'In every generation, one must consider himself as if he were exiled from Yerushalayim.' This year's mourning shouldn't be the same as last year's or years before it. Each year has its unique set of distress and challenges that we mourn for. This is as it states in the Kinos (#24) חדש מספד שנה כל אספוד ...המקדש בית חורבן על, "For the Churban Beis HaMikdash...I will mourn and eulogize each year anew."
Rebbe Pinchas Koritzer zt'l (Imrei Pinchas 378, 380) writes, "On this day, keviyachol, Hashem is crying. Therefore, when a person is also at this place [of mourning], he is protected. Laughing on Tisha b'Av is נפשות סכנת, dangerous because when one is with the king, he is protected, but when he is distant from the king, he doesn't have protection. And on this day, the Shechinah is keviyachol sitting on the earth [and the only way to connect and be with Hashem is through mourning]."
Surprisingly, Shulchan Aruch (559:4) states, "We don't say tachanun or selichos on Tisha b'Av because it is called moed, a holiday." It states (Eichah 1:15), מועד עלי קרא, that Tisha b'Av is a moed, a holiday. The question is, how can Tisha b'Av be called a holiday?
Reb Mordechai Gifter zt'l answers (quoting his Rebbe, hy'd) that מועד means a gathering place (like מועד אהל). Yomim tovim are called Moed because on yom tov, we gather and connect with Hashem through joy. Tisha b'Av is called moed because we connect to Hashem through our mourning.
On Tisha b'Av, we read איכה מגילת, and the word איכה is repeated at the beginning of many of its chapters. Furthermore, many of the kinos that we say on Tisha b'Av begin with the word אֵיכָה. In the Torah, Hashem asks Adam HaRishon, א ַיֶּכָּה, "Where are you?" Zera Kodesh (Devarim 'ג במדרש ה"ד) explains that throughout galus, Hashem asks א ַיֶּכָּה, "Where are you?" Hashem searches for us, wondering why we are so distant from Him. He searches until He finds us, as it states (Hoshea 9:10) מצאתי ישראל, "I found Yisrael." And when Hashem finds us, He sees that throughout the galus we were also constantly searching for Him. We were always asking, להעריצו כבודו מקום איה, "Where is Hashem so we can exalt Him?" Hashem and Bnei Yisrael search for each other until they unite. On Tisha b'Av, we unite through mourning; on yom tov, we connect through joy.
