Simcha!! JOY!! :)

The Rebbe gives many reasons why we should have simcha/joy:
Because G-d created us. Because He made us a Jew. He is always with us. He has given us so many things. We should be joyful that we have the ability to connect with Him. And we should be full of joy that we are heading towards moshiach.

To someone who complained about sadness the Rebbe wrote that one should be so busy doing what needs to be accomplished, that there be no time to think about sadness

The Zohar says that the way one acts in this world, is the way that he is dealt with above. If a person is happy and acts joyously, then his situation will improve.

The holy Tzaddik Reb Elimelech from Lizhensk would often perform various afflictions on himself as a kappara, one of which was rolling in the snow without warm clothes. One cold, winter night, while rolling in the snow, Reb Elimelech did not notice a nail sticking out from a board, for it was covered in snow, and he rolled over it, piercing his hand. When he arrived home, his relatives, seeing the hole in his hand, made a great tumult and each gave their advice on how to stop the blood flow. Reb Elimelech's daughter, overhearing bits of the discussion, thought they were discussing a hole in the wall, called out, "What's the big deal? Take some straw and stuff it up!" Hearing this, everyone began laughing, and suddenly Reb Elimelech stopped bleeding. He explained that there had been a decree passed above, but through the joy his daughter had caused, a joy had been aroused above, nullifying the decree.