Is it a Mitzvah to be Happy?

R' Nachman of Breslov notwithstanding, the answer is no.

Many, however, will quote from Devarim 28:47 to substantiate the claim that the Torah places a premium value on being joyful.

As usual, I think there's a common misunderstanding of this verse.

The verse reads:

תחת אשר לא עבדת את ה' אלהיך בשמחה ובטוב לבב מרוב כל

The common translation is (I'm filling in what's implicit, as it's obvious that this verse is an incomplete sentence):

[all of these horrendous things will befall you] because you didn't worship HASHEM your God with joy and good-heartedness, while you had everything.

Really, the verse begins a thought that continues in the next verse:

"And you will serve your enemies who God will send against you out of hunger, thirst, lack of clothing, and lack of everything; and he will place an iron yoke upon you intil he destroys you."

These two verses are a clear cause-and-effect tit-for-tat structure. You didn't serve God - therefore you'll serve your enemies. You had it good then - you'll have nothing now.

The translation of 'lo avadeta et H' Elokecha be-simcha' isn't "...Because you didn't serve God joyfully", rather, it's "Because you didn't serve God while you were joyful..." . The crime isn't a joyless service of God, rather, a joyful non-service.

A look at verse 45, 2 verses prior, reinforces this claim; it specifically points to non-observance as the cause for the punishment described.

As a final perplexing question, though, the misunderstanding of this verse seems to originate with Maimonides in the Laws of Lulav 8:15, where he writes:

שהשמחה שישמח אדם בעשיית המצוות ובאהבת האל שציווה בהן, עבודה גדולה היא; וכל המונע עצמו משמחה זו, ראוי להיפרע ממנו, שנאמר "תחת, אשר לא עבדת אתה' אלוהיך, בשמחה, ובטוב לבב" (דברים כח,מז).

The joy that one should enjoy when performing mitzvot and in loving God who commanded them is a great service. One who hold himself back from this joy deserves punishment, as it says "because...(our verse)"

I believe that Rambam doesn't mean that the punishment is reserved for one who doesn't serve God out of joy, rather, in context he's specifically referring to the Mitzvah of joy on holidays which deserves punishment, and the intent of the prooftext is purely homilletic (a drasha and nothing more).

Where do I get off saying that when the Rambam quotes a verse he doesn't really mean that it as the literal sense of the text?

Well, in this case, it's easy.

Rambam quotes this verse in two other places: Yom Tov 6:20 and Talmud Torah 3:13 . He understands this verse differently in each of those cases. In the former, he uses it to demonstrate that the only worthwhile joy is the joy that's actually in the service of God, and there's no mitzvah in empty joy on any holiday. In the latter, he understands it much in the way that I suggested, that this verse is the Biblical equivalent to the Rabbinic dictum that 'anyone who neglects the Torah while wealthy, will eventually neglect it out of poverty". Thus, Rambam quotes a single verse on 3 occasions in the Mishneh Torah, interpreting it differently and to different ends each time. Still think he meant it as pshat?

The implications of the verse, now that we have pshat, are wild. It condemns joy that is not somehow related to or accompanied by Divine service.