A person should sit down and think what it means that G-d commanded us to do a mitzva, never mind what the mitzva is, but just the fact that HE, G-d,, our Creator commanded US, His creation to do something for HIM.
The distance between G-d and us are the furthest points that one can imagine. Seemingly no connection at all.
And what did He do? He lowered Himself from His high lofty level and came down to US, human beings, and asked us to do certain things for him, Mitzvos.
The fact alone, that He lowered Himself to ask us to do something for him, that itself brings a connection between us and is reason enough for us to be b'simcha. To be joyful.
Imagine if the president of the United States would ask us to do something for him, it would create a closeness between us and we would be so happy. We would then, obviously perform the favor with love and great joy.
When we are b'simcha then our performance is much better too. The commandments that were asked of us are then easier to do. Our service to Hashem would then be different too. Like carrying a heavy sac of diamonds up a hill and not a sac of rocks.
(We are always wrestling with our evil inclination. The way to overcome him is to be in a good cheerful mood, to fight with a song, as is done when one goes to war.)
When Moshiach comes our Simcha will then be complete, for how happy we are now, we are still lacking one thing - the arrival of Moshiach.