In Devarim the Torah tells about different Regalim and the fundamentals of each Chag. It is here that the Torah issues the commandment of Simcha on the Regalim- a Mitzva that is enumerated among the 613 by both the Rambam and Sefer Hachinuch.
The Alter Rebbe poses a question. We learn in the Tochacha of Parshat Ki Tavo that one of the reasons for the curses is תהת אשר לא עבדת את ה" אלקיך בשמחה ובטוב לב, “Since you have not worshiped Hashem with Simcha and gladness.” From here we learn the concepts of מצווה גדולה להיות בשמחה , “A big Mitzva is to be happy.” From here the Rebbe asks: How can there be a specific Mitzva of Simcha on the Regalim? Is it not a Mitzva to be happy all the time?
The Lubavitcher answers as follows. The Mitzva of always being happy is to be happy while serving Hashem. If you are doing a Mitzva you should be happy while doing it. However, on Chagim the Mitzva of being happy gets elevates to a whole new level. On Chagim, happiness in itself is a Mitzva. Think about it- it is a Mitzva simply to be happy.
We can learn from this that Chagim are Hashem’s special gift to Am Yisrael. Our job on the Chagim is to be happy with ourselves, be happy with our lot as Jews, and most of all be happy with Hashem and his Torah as we say in the Amida on Shalosh Regalim וישמחו בך ישראל מקדשי שמך.