As Meryl says, (there -- she said it again!) "They tried to kill us, they failed, let's eat."
A man is obligated to imbibe on Purim until he can no longer distinguish between "Cursed is Haman" and "Blessed is Mordechai."
Purim is a veritable cornucopia of paradoxes which ignite the imagination of both scholar and layman. But perhaps the greatest challenge of all is posed by this requirement to indulge in drink to the point of losing the faculty of discernment. How, ask the commentaries throughout the generations, can we be commanded to invite that very intoxication which is so roundly reviled in both Scripture and Talmud? And why such a puzzling standard of non-discernment?
Good questions, and this explanation is wonderful, but too long to quote here. Go and read.
Purim Sameach!
