A thought from my beloved friend R' Daniel Zweigbaum:

There are certain mitzvos that exist, which show Hashem's surreal level of generosity. Mitzvos that most people have the desire to fulfill due to the human nature which Hakadosh Boruch'Hu instilled within them. but Hashem, in His endless love for us, the chosen nation, decided to give us schar for doing them! (i.e. Getting married, respecting ones parents, giving tzedaka...etc). I would like to suggest that perhaps the mitzvah requiring us to be b'simcha on purim goes along the same idea. If we truly internalized the message of the megillah, and believe that Hashem is ALWAYS behind the scenes, then of course on the day of purim we're going to be happy! For if we genuinely realized that everything is caused by Hashem, there is no such thing as bad anymore, only what we perceive to be bad! From this perspective the mitzvah of being b'simcha on Purim is just another example of how badly Hashem wants us to succeed in our goal of developing our connection with Him, and fulfilling His ratzon.