Why do we strive to achieve a state in which we can no longer differentiate between things which are so diametrically opposed:Haman and Mordechai?
It is as if we are saying: "Even if we can no longer differentiate between things whose differences should be abundantly clear, we still know that we shall not lack salvation, that our hopes are not fruitless and that our joy is not unbased, for in G‑d alone do we place our trust. Whether sober or inebriated, we fear no evil, for You are with us forever."
The Sages offered a number of other allegorical and mystical explanations for the mitzvah of drinking to the point of being unable to differentiate.
Anyone can differentiate between "cursed is Haman" and "blessed is Mordechai." A person, however, must learn to recognize the various intermediate stages that are between these two extremes so that he can determine which matters incline towards good and which incline towards evil.
If one had drunk enough on Purim so that these intermediate stages are no longer clear, he is considered to be sufficiently intoxicated to have fulfilled the obligation. Alternatively, there are two ways in which holiness finds expression in this world: either through the victory of the just or through the downfall of the wicked.
Our Sages said: Just as G‑d's praises are expressed by the righteous in paradise, so too are they expressed by the wicked in purgatory. However, G‑d, as it were, prefers the praises offered by the righteous.
When Israel acts meritoriously, the righteous are exalted and it is their praise that is expressed; all are happy and the joy is complete. But when Israel lacks merit, their salvation is realized through the downfall of the wicked who are excessively evil and descend to purgatory. The entire world trembles in fear of G‑d, but there is no joy. Thus, the happiness of "blessed is Mordechai" - of Israel being saved through her own merits - is greater than "cursed is Haman" - the salvation that comes when the wicked have been destroyed.
Nevertheless, the Sages ruled that on Purim one is required to drink until he reaches the point where he can no longer differentiate between these two types of salvation. Why? Because the downfall of Haman is completely different from the downfall of other wicked people. The joy that results from his defeat is as complete as that which results from the victory of the righteous. Haman is a descendant of Amalek, of whom the verse states: “And in the destruction of the wicked there is song (Proverbs, 11:10)”. When Amalek is obliterated, it is as if there is a revelation of the Shechinah in the world and it is therefore fitting that we celebrate.
Thus, there is no difference between the joy associated with "cursed is Haman" and that associated with "blessed is Mordechai." So that man might not be distressed that he has merited salvation because of the excessive evil of the wicked rather than through his own merit, our Sages ordained that he drink and forget the difference between these two sources of salvation.