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?
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.
In the Gemara it says: "לבסומי בפוריא". Which is commonly translated as drink. However it's clear that לבסומי is actually a reference to the word סמים which means drugs.
Other translations for לבסומי are "sweet things", and "sing agreeably". Ref
Clearly the intent of the Gemara is that a person should make brownies laced with drugs, and eat them until they spontaneously sing.
This conveniently fulfills all the dictums of Purim at once: Drinking, feasting and making merry.
Warning: The following answer contains elements of actual Torah and is not meant to be taken entirely in jest.
It says in T'hillim (19:9): פִּקּוּדֵי יְהוָה יְשָׁרִים, מְשַׂמְּחֵי-לֵב. The Radak comments that Torah gladdens the heart, namely the intellect:
הם משמחי לב, כי החכם ישמח על שכלו. וכאשר יגבר על הגוף וינהגהו בדרכי השכל, אין שמחה בעולם כשמחה ההיא, והיא שמחת הנפש. לפיכך אמר: משמחי לב, ולא אמר משמחי האדם, כי האדם ישמח לתאות העולם, אבל השכל, והוא הלב, לא ישמח כי אם בדרכי שכל
Yet, we see from T'hillim (104:15) that wine gladdens the heart, and the reference here does not seem to be to the intellect: וְיַיִן יְשַׂמַּח לְבַב אֱנוֹשׁ.
What's the difference? The mishna in B'rachos (9:5) expounds: בכל לבבך בשני יצריך ביצר טוב וביצר רע. Thus we see that לְבַב indicates that wine allows one to gladden the totality of their being, including the yetzer hatov and the yetzer hara.
The gemara (M'gillah 7b) writes that on Purim one should drink festively: מיחייב איניש לבסומי בפוריא עד דלא ידע בין ארור המן לברוך מרדכי. However, because of the unfortunate incident recorded immediately after this, many rishonim and acharonim follow the Orchos Chaim and rule we don't have to get sooooo drunk. Still, they write, one should drink יותר מלימודו, more than his studies.
Why does the drinking have to exceed the learning? The rest of the year, we serve HaShem with joy by achieving שמחת הנפש, with our יצר טוב. However, on Purim we fulfill עבדו את השם בשמחה in the manner of לאהבה את השם א-להכם ולעבדו בכל לבבכם , with the entirety of our being. Why is Purim a special opportunity for this?
In the time of the Purim story, the Jews attended the feast of Achashveirosh. The feast was devoted to celebrating the ruins of the Jewish Temple and degrading its holy items. Achashveirosh himself brazenly wore the priestly vestments. The Jews should have sensed with the totality of their being that something was amiss; the celebration of Achashveirosh should have disturbed a Jew to his very core. Instead, what was the focus? The fact that technical halachos about the food at the seudah were observed (Megillah 12a). For this reason, that generation of Jews was nearly destroyed (ibid.).
In those times, the Jews corrupted their Torah learning by using it to justify gladdening themselves in a blasphemous celebration. We rectify that error by drinking festively more than we study; we allow the totality of ourselves to emerge free from the restraints of the intellect and joyously serve HaShem with שמחה, with all our heart, בכל לבבנו בשני יצרינו.
So it's well known that you must drink on Purim - however it does not specify what you must drink.
On every day of the year you must drink wine (אלא ביין....) but on Purim you must drink water till you experience water intoxication.
Some of the symptoms of water intoxication are confusion and drowsiness, which fulfills the dictum of עד דלא ידע.
Yayin-70 in Gimtarya "Shivim Panim LaTorah" really Yayin is referring to Torah and therefore one should always be immersed in Torah and then they will be happy.
