How do we understand the concept of serving Hashem with joy? 

Is there really a misvah gedola lihiyot besimcha tamid? 

How do we define simcha ? 

David corrected Michal his wife, daughter of Shaul's misconception concerning the meaning of simchah in the service of Hashem. He
emphasized that this type of simchah must be expressed physically
and outwardly—even if it means that one must seemingly act beneath one’s dignity for the sake of Hashem— and display his simchah like a commoner.  

 Zohar hakadosh (Toldos 138a): “אצטריך יצר הרע לעולם —כמטרא לעולם, דאלמלא יצר הרע חדוותא דשמעתא לא ליהוי” the world requires the yetzer Hara just like it requires rain; if not
for the yetzer Hara, simchah related to Torah study would not
exist. What a powerful and incredible idea! 

Based on the Shvilei Pinchas: The yetzer Hara yearns to be transformed into a force for good, so that it, too, can participate in the service of Hashem. Yet, when people do not serve Hashem with simchah, it remains in its original evil state. In fact, it is angry
with man for not having rectified it and transformed it. As
revenge, it prevents man, as well, from elevating himself in the
service of Hashem; it rallies all of its forces to bring man down
spiritually by means of its various traps. These are the very
same forces that man, had he been wiser, could have transformed
into instruments for good by simply performing misvot
with simchah.