simchah joy happiness - Is 'simcha' a mitzvah?

"Mitzvah gedola le'heyos be'simcha..." many of us have heard this. I am wondering if there is an actual mitzvah to be be'simcha? (I am looking for sourced comments and not just idea's). I know that simcha is not counted as a mitzvah by the Rishonim, but I think the question still stands. I have heard that the Chasam Sofer (in parshas VaYechi) says that it is a mitzvah deoraysah and explains why the Rishonim don't count it, but I couldn't find it.

Answer

Which joy

Rabbenu Bahya writes in Kad HaQemah (Simha) that we don't find any sources which praise joy except for joy in serving God:

אין השמחה ראויה בעוה"ז כי אם בעבודת הש"י ולא תמצא בכל ספרי הקדש שישבח השמחה אלא בענין העבודה וההשגה בהקב"ה

Rambam writes in Hilkhot De'ot (1:8) that one should be happy all the time:

ולא יהא מהולל ושוחק, ולא עצב ואונן, אלא שמח כל ימיו בנחת, בסבר פנים יפות

Rambam in Hilkhot Yom Tov (6:19) writes that we are only commanded in joy that that includes service of God:

ולא נצטווינו על ההוללות והסכלות, אלא על השמחה שיש בה עבודת יוצר הכול

At the end of Hilkhot Lulav (8:15) he writes that joy in serving god is a great avodah (divine service):

שהשמחה שישמח אדם בעשיית המצוות ובאהבת האל שציווה בהן, עבודה גדולה היא

So although Rambam writes that one should be happy all the time, it seems that the primary joy that one is encouraged to have is joy in the context of religious service, as Rabbenu Bahya wrote.

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Source for the quote

The quote itself is found in Likkutei Moharan (Tinyana Torah 24) who phrases it in terms of Galen's humours:

מצוה גדולה להיות בשמחה תמיד, ולהתגבר להרחיק העצבות והמרה שחורה בכל כחו

Significantly, and less frequently quotes, he clarifies there that one should set aside a time of the day to be brokenhearted and pray to God:

אף שגם לב נשבר הוא טוב מאד, עכ"ז הוא רק באיזו שעה. וראוי לקבוע לו איזה שעה ביום, לשבר לבו ולפרש שיחתו לפניו

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Is it a mitsvah

"I know that simcha is not counted as a mitzvah by the Rishonim":

Actually, Rabbenu Bahya writes in multiple places that joy is a mitsvah...When it is joy for mitsvot. For example, he writes in his commentary to Deuteronomy (28:47):

והשמחה במעשה המצוה מצוה בפני עצמה

The joy in doing a mitsvah is a mitsvah in and of itself.

You are correct that the vast majority of Rishonim, such as Rambam, do not count it as its own technical mitsvah. However, as we quoted from him, he nevertheless states that it constitutes a great act of divine service