In Parshas Re'eh, the Torah refers to simcha several times. It is striking, when the Torah refers to simcha, one finds the phrase “lifnei Hashem.”  The Torah writes[i] “v'samachta lifnei Hashem Elokecha,” “you should have simcha in front of Hashem.” Earlier[ii], the Torah writes, “v'achalta sham lifnei Hashem Elokecha v'samachta ata u'vei'secha.” You find this repeated elsewhere as well[iii]. In Chumash Devarim, nearly every time the word simcha is mentioned, you will find the words “lifnei Hashem” next to it, either right before of right after. What is the p'shat? What message is the Torah teaching us?

Rav Soloveitchik zt”l, the Rav, once developed the following theme[iv]. We see here in the Chumash a basic yesod of Jewish hashkafa. In Yahadus, simcha means being “lifnei Hashem.” That is what simcha is. Simcha means being “lifnei Hashem,” being in the presence of and feeling close to Hashem. There are various mekoros that reflect this theme. For example, the Gemara (Chagigah 5b) says, “ein atzvus lifnei HaKadosh Baruch Hu,” “there is no sadness in front of Hashem,” as it is written[v] “hod v'hadar li'fanav oz v'chedva b'mkomo.” There is only simcha in front of Hashem.

The Rav explains that we know that Yom Kippur is a yom simcha. The Rambam[vi] discusses the reason why we do not say hallel on Rosh Hashanah and Yom Kippur. He says that they are not yimei simcha yi'seirah because they are days of teshuva, yirah, and pachad. The Rav pointed out that it is meduyak in the Rambam that although there is not abundant simcha on Yom Kippur, there still is a mitzvah of simcha on Yom Kippur. We do not express it with hallel, but there is simcha. The truth is, the Mishnah[vii] already says that Yom Kippur is a very happy day- it was one of the days on which marriages were arranged[viii]. Yom Kippur is a yom simcha.

What is the p'shat that Yom Kippur is a yom simcha? The Gemara near the end of Ta'anis says that one of the reasons that Yom Kippur is a yom simcha is because it is a time of selicha u'mechila; Hashem forgives you for your aveiros. The Rav explains that aveiros are mavdil between us and Hashem, they act as a barrier between us and Hashem. The posuk reads[ix], “ki im a’vonoseichem ha'yu mavdilim bei'nei'chem u'vein Elokeichem.” Aveiros separate us from Hashem. When a person does teshuva and is forgiven for his aveiros, then he is able to be close to Hashem. It is a very beautiful p'shat. A person does teshuva for his aveiros, and he does not eat and drink and adds ruchniyos to his life, and he becomes closer to Hashem! That is the simcha of Yom Kippur! This is the simcha of being “lifnei Hashem.”[x]

This is a major theme in Yahudus. Being b'simcha means being “lifnei Hashem.” That feeling of being “lifnei Hashem” brings the greatest simcha, and that, the Rav explains, is one explanation as to why the Torah nearly always puts the phrase “lifnei Hashem” next to the word “simcha.”

As Yom Kippur is about to begin, we say Kol Nidrei.  Then we say two pesukim in which we ask Hashem to forgive us for our aveiros, “v'nislach…” and ‘s'lach nah…” and finally we say the brocho of shehecheyanu. The Rav explained[xi] that the flow of our davening- from pesukim leading into the shehecheyanu- might be based on the p'shat developed above. The brocho of shehecheyanu represents the simcha of the day, the simcha of the yom tov. Why do we say these pesukim of “v'nishlach l'chol adas B'nei Yisroel” right before the shehecheyanu? Because this is the nature of the simcha of Yom Kippur. When we are forgiven for our sins, we become closer to Hashem, and that leads to the simcha of Yom Kippur. We are about to say shehecheyanu, and therefore we introduce shehecheyanu with the pesukim of selicha u'mechilah because that is exactly the nature, that is the yesod, of the simcha on Yom Kippur. This is a beautiful explanation of the Rav[xii].

This is the Rav's approach to the idea of simcha. Simcha means the experience of being “lifnei Hashem.”  Every time we do mitzvos, we become closer to Hashem. We should learn to feel and appreciate the simcha of mitzvos, the simcha of being “lifnei Hashem.[xiii]”  

[i] Devarim 16,11
[ii] Ibid. 14,26
[iii] See the Concordantsia for other examples of simcha in Chumash Devarim, and you will find lifnei Hashem in those pesukim as well.
[iv] See Nefesh HaRav of my rebbi, Rav Schachter shli”ta (p.314-315) and Shiurim L’zecher Abba Mori z”l (vol.2, p. 194-5).
[v] Divrei Hayamim 1:16,27
[vi] Hilchos Chanukah 3,6.
[vii] See the end of Mishnayos Ta’anis.
[viii] The Mishnah describes two days on which they made shidduchim-  Tu B'av and on Yom Kippur.
[ix] Yeshaya 59:2
[x] We know that the Kohen Gadol is not me'tameh himself if a relative dies, he is supposed to remain b'simcha. The Gemara (Moed Katan 14b) explains, “kol ha'shana k'regel...,” “the whole year is like a regel for the Kohen Gadol.” That is the language of the Gemara. The language of “k'regel” sounds like there is some idea of simcha all year round. What is the p'shat of the Kohen Gadol being  b’simcha all year round? The Rav said that the p'shat of the simcha of the Kohen Gadol is that the Kohen Gadol was always “lifnei Hashem.” He quotes the Rambam in Hilchos K'lei Hamikdash 5:7. The Rambam says, “u'bayis yi'hi'yeh muchan b'mikdash,” “he had a home in the beis hamikdash,” “v'tifarto k'vodo she'yoshev b'mikdash kol ha'yom,” the Kohen Gadol would sit in the beis hamikdash the whole day, he would always be there. “V'lo yei'tzei e'lah l'veiso ba'lai'lah,” “he would not leave except for when he went home at night,” “v'yi'hi'yeh bei'so b'Yerushalayim v'ei'no zaz mi'sham,” his bayis had to be in Yerushalayim.

That could be the explanation here. The Kohen Gadol was always “lifnei Hashem;” he was either in the beis hamikdash or in Yerushalayim- b'davka in Yerushalayim. The Kohen Gadol was not allowed to leave Yerushalayim. Why? He has an obligation to always be “lifnei Hashem;” and that is the simcha of the Kohel Gadol all year round.

Aveilus and simcha do not coexist. This is why the arrival of a Yom Tov cancels the remainder of the aveilus period. The Kohen Gadol always has a chiyuv of this simcha and therefore is not noheg aveilus. 
[xi] See ‘Rabbi Soloveitchik on The Days of Awe’ p.116-118.
[xii] There is a special mitzvah of simcha of a chosson and kallah during the days of sheva brochos. I thought that maybe one could say that this is an angle on the simcha of a chosson and kallah. The Gemara says that “ish” and “isha” both have the letters aleph and shin. Then, “ish” has the yud and “isha” has the hey. The yud of the “ish” together with the hey of the “isha” form Hashem's name. Chazal say that if they are zocheh, “ha'shechinah sh'ar'uy bei'nei'hem.” Maybe this is the idea- a couple gets married and there is now the potential for the shechinah to be in their lives in a more powerful way. The couple gets married and “ha'shechinah sh'ar'uy bei'nei'hem,” so there is simcha there. Perhaps that is one aspect of the simcha of the marriage of a chosson and kallah; the simcha of experiencing the shechinah in a new way which one was not able to experience before.
[xiii] The Torah tells us in Parshas Ki Savo that one of the reasons that the terrible k'lalah came upon us is, “tachas asher lo a'va'd'ta es Hashem Elokecha b'simcha u'v'tuv le'vav,” because you did not serve Hashem b'simcha. Some meforshim say that this posuk is teaching us that aside from the chiyuv to do mitzvos, there is another chiyuv to do mitzvos b'simcha; that is what the posuk means. “Tachas,” “because” you did not serve Hashem b'simcha, that is why Hashem is sending this k'lalah. (Devarim 28,47). Rabbeinu Bachye learns the posuk this way, and the Rambam quotes this posuk in Hilchos Succah regarding the Simchas Beis Hashoe'vah. Not only are we obligated  to do mitzvos. But, there is also a mitzvah to do mitzvos b'simcha..

What is the p'shat?  I thought, based on what the Rav said, one could say a very simple idea. A person is always “lifnei Hashem.” But, when a person does a mitzvah, he is mamash experiencing a special closeness of“lifnei Hashem;” you are doing what The King says, you are in The King's presence. Therefore, one can certainly say that when you do a mitzvah you are closer to Hashem and more in Hashem's presence. That is the p'shat. Every time you do a mitzvah you should feel, “I am closer to Hashem now!” and that itself should bring simcha. If a person does mitzvos without simcha, it shows that he does not really appreciate that he is in the presence of Hashem or that he is keeping Hashem's Torah and mitzvos. That might be the idea of the special inyan of doing mitzvos b'simcha.