If I am obligated to be drunk all the time, how is Purim different?

We all know the halacha of עִבְדוּ אֶת-יְה-ה בְּשִׂמְחָה (Tehillim 100:2). But the gemara paskens (Pesachim 109a) that since the destruction of the Beis Hamikdash "אין שמחה אלא ביין" ("Joy only comes from wine"). So obviously, one must be drunk whenever serving G-d. Lest one think that there are some times one can be sober, the Mishna (Avos 2:19) tells us "לא עליך כל המלאכה לגמור" (that one may never stop serving G-d).

Clearly, one is always obligated to be drunk.

So how is Purim different?

Answer

Clearly אין שמחה אלא ביין is not talking about joy, since we can experience joy in many ways. Rather, it is referring to שמחה the man. Simcha was a notable drunk who would go to every party that was thrown. The sages realized that if only people would stop serving alcohol, Simcha would not come and be a nuisance.

This is the famous rowdy Simcha who the sages complain after every wedding, long after the party was over:

עוֹד יִשָׁמַע בְּעַרֵי יְהוּדָה וּבְחוּצוֹת יְרוּשָלַיִם

קוֹל שָשוֹן וְקוֹל שִמְחָה קוֹל חָתָן וְקוֹל כָּלָה

[Oy! The party was over five hours ago already, and] we can still hear in the streets of Judah and the avenues of Jerusalem

The shouts of Simcha, Sasson, the groom and the bride!!!

[Haven't they partied late enough?! People have to work the next day!]

There are those who say that Simcha simply was very seriously taking on the mitzvah of rejoicing with the bride, and in honor of that opinion, we bless Sasson, Simcha, and all their friends in all weddings:

ברוך ... אשר ברא ששון ושמחה, חתן וכלה, גילה רינה, דיצה וחדווה, אהבה ואחווה, ושלום ורעות

Blessed are You... Who creates Sasson, Simcha, the groom, the bride, Gilah, Rinah, Ditzah, Chedvah, Ahavah, Achvah, Shalom, and Re'ut.