The Kabbalah teaches us this all important principle of Jewish mysticism:
One side of your heart should be singing, the other side crying!
Often in life we are either singing or crying!!!
The problem with this is as follows, when we sing we forget that others cry and when we cry we forget that others sing.
In order to overcome this and as the Rebbe says, that God gives us the power to have opposites simultaneously, a Jew must must MUST! contain these two opposing elements!
In every generation the great mystics taught us how to!
In our generation the Rebbe taught us how as well!
In a talk regarding the greatness of the Jewish women and girls of our generation he writes that on one hand we must be singing that Moshiach is so close (as we see from the myriad messianic transformations which have begun – more peace in the world – more God consciousness – more returnees to Judaism – more revelation of the Kabbalah/Chassidus in anticipation of the messianic era when this will be the primary study which will flood the worlds appetite for divinity and more!) and on the other hand we must be sad that Moshiach has not come (and hence there is much sadness in the world.)