Yesod v'Shoresh HaAvodah (p.199 from the older edition) writes, "The primary service of man to Hashem is the joy in his heart for Hashem and His mitzvos. This joy can be felt at any time and any moment and no one knows about it but Hashem, Himself. And this is the primary avodas Hashem.
The importance of joy for the mitzvos is expressed in the tochachah of parashas Ki Savo, as it states (Devarim 28:47) תחת אשר לא עבדת את ה' אלקיך בשמחה ובטוב לבב. This means that the entire tochachah came to be because we didn't serve Hashem with joy.
The Chareidim writes, "The Arizal revealed that everything he perceived, his ruach hakodesh, and great revelations in the secrets of Torah, were all a result of the endless happiness with which he performed every mitzvah."
It states (Shir HaShirim 3:11) ביום חתונתו וביום שמחת לבו. The Zohar (Chadash, Bereishis 15:) asks, "What is יום חתנתו? It is the day one can perform the mitzvos of the Torah, for that day is a great joy for tzaddikim. When is that day? It is when he becomes thirteen. It is an obligation for tzaddikim to celebrate that day like the day that one goes to the chuppah."
The Chasam Sofer writes, "I said that happiness is the first mitzvah a bar mitzvah bachur performs when he becomes bar mitzvah. It is his joy to accept the yoke of heaven. This joy is a mitzvas asei, a positive command of the Torah, for we must serve Hashem בשמחה ובטוב לבב מרוב כל. I explained that the Torah doesn't explicitly write that we should rejoice on the day one becomes bar mitzvah, just as the Torah doesn't write explicitly that we should be happy on Shavuos, the day we receive the Torah. This is because the Torah can't command us to be happy because perhaps the mitzvos are perceived by the person as a yoke. And then the mitzvah to be happy will be yet another yoke on our shoulders. Rather, Hakadosh Baruch Hu doesn't command it, and we are happy on our own, happy with receiving the yoke of Torah and happy with the bar mitzvah."
People make a grand celebration by a chasunah, which is the fulfillment of the first mitzvah of the Torah (פרו ורבו), and at a hachnasas sefer Torah, which is the final mitzvah of the Torah (ועתה כתבו לכם את השירה הזאת). The Chidushei HaRim zt'l explains that we should really make a great celebration for each mitzvah we perform, but it would be impossible. Therefore, we make a celebration solely for the first and the final mitzvah of the Torah.
Another pasuk that speaks about our obligation to serve Hashem with joy is (Tehillim 100:2) עבדו את ה' בשמחה. The Rambam (Hilchos Lulav 8:15) states, "The joy that a person has when he performs a mitzvah, and his love to Hashem Who created him, עבודה גדולה היא, is a great avodah, a great service to Hashem."
Each mitzvah has its segulah, and yet, sometimes, we perform mitzvos but don't see the segulos coming through! Reb Chaim Vital zt'l (Hakdamah l'Shaar HaMitzvos) addresses this problem and explains why. He writes, "We see people performing mitzvos, but we don’t see the words of Chazal, which discuss the reward even in this world, happening. The primary factor for this is that when one does a mitzvah, he shouldn't consider it like a yoke, something that he wants to finish as soon as possible. Instead, he should think that he is earning millions (אלף אלפים דנרי זהב) with every mitzvah he performs. He should be very happy with the mitzvos, and perform them with great desire, as though he is actually earning the millions for each mitzvah he performs."
Similarly, Orchos Tzaddikim (Shaar HaSimchah) writes, "Whoever performs a mitzvah with joy, his reward will be a thousand times more than those who perform the mitzvos as though they are a heavy load."
The Gemara (Shabbos 30:) states that the Shechinah resides solely where there is joy. And the Vilna Gaon zt'l (Oros HaGra, Simchah) writes, "Atzvus is a mum, a blemish, and whoever has a blemish, can't come to kedushah. This is the reason one needed to be happy in order to come to the Beis HaMikdash."
How can one acquire this great joy? A person should think about this tremendous opportunity that he, a human being, can serve Hakadosh Baruch Hu, the King of all kings. The Orchos Tzaddikim (Shaar HaSimchah) writes, "When you perform the mitzvos, be happy in your heart that you have the merit to serve the Divine King, to Whom the malachim bow down."
To better explain the great joy that is required when performing mitzvos, let us look at the Or HaChaim HaKadosh (Shemos 12:3) on the pasuk ויקחו להם איש שה, which is talking about the sheep that the Yidden took for the korban Pesach. In this pasuk, the word איש seems extra. איש represents Hakadosh Baruch Hu, as Chazal (Sotah 42:) tell us אין איש אלא הקב"ה. This was the Yidden’s first mitzvah, and at this time, Hashem wanted to show them what they would attain when they performed the mitzvos. ויקחו איש, it will be like they are taking Hakadosh Baruch Hu because when a Yid performs a mitzvah, Hashem resides on the Yid who performs it. This is also the intention of (Shemos 25:2) ויקחו לי תרומה... The word לי is extra, and the Midrash Tanchuma (3) writes that this means we should take Hakadosh Baruch Hu with the mitzvah. For this is what occurs when we perform the mitzvos. We become connected with Hakadosh Baruch Hu. This is also the intention of the pasuk (Devarim 28:10) וראו כל עמי הארץ כי שם ה' נקרא עליך, because of the mitzvos, it will be noticed that Hashem resides with Yisrael.
Thinking about these wondrous ideas will undoubtedly fill our hearts with joy when we perform the mitzvos.