The Rebbe explains that since a Jew is constantly serving G-d in all he does then he needs to always be joyful.
Our sages say that the Shechina, G-d's Holy Presence, does not rest on those who are sad or downhearted. It only rests where the joy of a mitzva is present.
When the prophet Elisha became angry at Yehoiram for his wicked ways, the spirit of prophecy left him. Only after music was played before him, did the spirit of prophecy return.
Walking around the marketplace, the Amora Rav Broika asked Eliyahu Hanavi if anyone who was there was meritorious of Olam Haboh,(the world to come), and Eliyahu Hanavi answered in the negative. Soon two brothers entered the marketplace, and Eliyahu Hanavi pointed to them, saying, "These will merit Olam Haboh." Rav Broika approached them and asked them how they conduct themselves. "We are joyful people and we make those who are sad, happy. If we hear about an argument, we make peace, using humor, between those quarreling."
The Rebbe explained that Yidden should rejoice out of trust that G-d will bring moshiach soon. This will then hasten G-d, so to speak, in sending moshiach."