Bamidbar

The Torah describes how the Jewish people should encamp in the desert: איש על דגלו באתת לבית אבתם יחנו בני ישראל מנגד סביב לאהל מועד יחנו , The Bnei Yisroel shall encamp, each man at his division according to the signs of his father’s house, at a distance surrounding the Tent of Meeting shall they encamp.
The בן איש חי says we could learn the importance of doing the mitzvos with joy from here.

Two "shin's” are embedded on the Tefillin; one is a regular one with three heads & one with four. The one with three is to invoke the merits of the three Avos & the one with four to invoke the merit of the four Imahos. Now if one were to take the two Shin's & put them together you get the word שש, joy, teaching us that the mitzvos need to be done joyfully.
Only then will we merit the final redemption.  As the posuk says, כי בשמחה תצאו , with joy you will go out, meaning go out of the golus. In the same vein, Hashem admonishes the Jews אשר לא עבדתי בשמחה , because you didn't serve me joyfully;; since happiness is one of the most important aspects of the mitzvah.

Why put the word for joy specifically on the tefillin but no other mitzvah? Since the teffilin represent the readiness of the heart & head to serve Hashem, it is fitting for the tefillin to also hint to us the importance of joy.

Now, how do we see this from the posuk? Says the בן איש חי , when the Jewish people do the mitzvos  באתת לבית אבתם , according to the signs of their fathers’ house, meaning the אותיות that are embedded in the tefilllin that hint at the forefathers & mothers. If the Jews will take from them the knowledge that the mitzvos have to be done happily, then  מנגד, break up the word; the first two letters, מן “from”, the ג , three and ד , four  heads on the "shin" that symbolize our forefathers & mothers, then,  סביב לאהל מועד יחנו , surrounding the Tent of Meeting shall they encamp & we will have the ultimate redemption.  

On a practical level, how do we get to this level of simcha for mitzvos?
Reb Aharon of Karlin says that the first words of the Posuk איש על דגלו teaches us that each person was born to fulfill a specific role; there was no one ever like him, nor will there ever be one like him that could do what he could do. Each person is special; designated as the one & only that could fulfill the plan that Hashem has set for him.

Therefore, each person has to camp under his banner doing what he is supposed to do & not try to do what someone else is doing. If each person would just try to do what he is good at, without looking at what other people are doing & feeling the need to live up to others, then everyone would be truly happy with his/her service to Hashem. No one would feel lacking because he’s not accomplishing what the other person is , and that joy will merit the coming of Moshiach speedily in our day!