1) Because it's a good way to get things done. To quote the Chassidic
classic Tanya, by Rabbi Schneur Zalman of Liadi (1745-1812): "Just
like in the case of two people wrestling, each trying to throw down the other,
if one of them moves with sloth and lethargy, he will be easily defeated and
felled, even if he is stronger than his fellow. So, too, in battling one's evil
inclination, one can prevail over it ... only with alacrity that comes from joy
and from a heart that is free and cleansed from every trace of worry and
sadness." Applies to wrestling, moral battles and everything in between.
2) Because it's a good thing to do. Why should joy be just a tool, a
means to an end? It's a good thing in its own right, a better way to be. And
it's not that difficult to achieve. Just focus on all the good things you have
and are a part of, and on how much more real and enduring they are than the
not-such-good things. So even if the latter are taking center stage in your
life, they don't belong there. Push them off and bring on the real players.
3) Because it's a happy time. Being happy sometimes takes an effort to
achieve, as in reason #2 above. But there are times when happiness is in the
air, and all you need to do is open yourself to it and allow it to enter your
soul. We're now in such a time. Our Sages tell us that, "When the month of
Adar enters, joy increases." As Haman unhappily (for him) discovered, it's
a time when good things happen to the Jewish people. You don't have to do
anything to experience it -- just don't shut it out.
4) Because it's what you are. This one is not really a
"reason", so I guess that means there are really three reasons, not
four. The Chassidic masters tell us that our soul is "literally a
part of G-d." So joy, ultimately, is not a technique to master, nor a goal
to achieve, nor even a state to surrender to. It's what we are, by virtue of our
bond with the One whom "Strength and joy are His place" (I
Chronicles16:27). Why hide from what we are?