Vehaya eikev tishm'un eis hamishpatim ha'eileh..  veshamar Hashem Elokecha lecha es habris v'es hachesed asher nishba la'avosecha.
"And it will be if you listen to these statutes.. and Hashem your G-d will guard for you the covenant and the kindness that He swore to your fathers."

The Ba'al Haturim points out that gematria, the numerical value of the word eikev (ayin=70,kuf=100, bais=2) is 172.  This is the number of words that comprise the first aseres hadibros, the ten commandments (in Parshas Yirso).  If we keep the ten commandments, we will merit the "great good that You have hidden away for those who fear You."
In the preceding Parsha, Va'eschanan, which we read just last week, the aseres hadibros, the commandments are repeated.  The number of words there is not 172.  Why does the Ba'al Haturim mention this calculation here?

The Pri Tzaddik counted the words of the second set of dibros, and found an extra 17 words, numerical value of tov, good (tes=9, vav=6, bais=2).  This corresponds to what the gemara (Bava Kama 55) says,

"Why do we not find the word "good" in the first commandments, and in the second, we do? (In Parshas Va'eschanan, the fifth commandment the Torah states, l'maan YITAV lach, "in order that is be good for you."  This clause is not found in Parshas Yisro.)

Further on in the Parsha, Moshe Rabbainu recalls the cheit ha'egel, how he was given the first set of luchos, and cast them down when he saw what had transpired.
Ki yagorti mipnei ha'af vehachaima asher katzaf Hashem aleichem l'hashmid eschem..
"For I feared from the anger and the wrath that G-d was angry at you to destroy you.."

The Shaim miShmuel queries: why did Moshe Rabbainu fear the anger of Hashem at the end of the first set of forty days, when he descended from the mountain?  Didn't Hashem already tell him about the nation's terrible deed, and didn't he already pray and accomplish the nullification of the decree?  In Parshas Ki Sisa in Sefer Shmos, that is the order of events.  Hashem informed Moshe about the sin before Moshe descended from the mountain, and Moshe prayed that Hashem remember the Bris Avos, the covenant with our forefathers, and that He not carry out the decree.  The Torah then says, Vayinachem Hashem al haraah asher diber laasos l'amo, "Hashem revoked the evil decree which He had spoken to do to His nation."
Why, then, was Moshe Rabbainu so terrified once he came down the mountain???  He had already been informed by Hashem of the sin he was to witness!  What did he see that so provoked his concern, that he smashed the luchos???

Vayehi kaasher karav el hamachaneh vayar es ha'egel umcholos vayichar af Moshe vayashlech miyado es haluchos vay'shaber osam..
"And it was when he came close to the camp and he saw the calf and the dancing, and Moshe was angered and he threw from his hands the tablets and he broke them.. "

HOWEVER, once Moshe came down, he saw something that caused him to despair that the former situation could be restored at that time.  What he saw was the dancing, the joy and the frivolity.  Simcha results from profound impression and emotion that permeates and includes all levels of the soul.  These dances were very very bad.  The sinful dancing created two more angels of destruction, called af and chaima, anger and wrath.  It seems that when Moshe was still on the mountain, Hashem did not inform him of the presence of these two frightful angels, for the mcholos had not yet taken place.  Even though Hashem had already recalled the decree, the situation took a grave turn for the worse because of the dancing.  The merit of the Avos was not enough to restore the former situation then.  Now Moshe had to contend with these two new angels of destruction.  The rest is history; he then shattered the luchos, and had to pray another two sets of forty days, when he received the second set of luchos, and was taught the Torah sheb'al peh.

From this we can learn how precious is simcha shel mitzvah, being enthusiastic and joyful and excited when we perform mitzvos, and chas veshalom, how harmful is misplaced joy.  We can now understand when the Rashba says that we have a tradition that whoever observes and participates in non-Jewish impure cultural events and entertainment will not take part in the "hunt for the Livyasan" in the future days.

Why are these two days such joyous days?  The gemara(Taanis 30) says that the reason for joy on Yom Kippur is that this is the day that the second luchos were given, and cheit ha'egel was atoned for.  The gemara gives a list of reasons for the joy on Tu b'Av (15th of Av). On this day:

1. The tribes were permitted to intermarry one generation after entering the Land.
2. The ban on intermarrying with the tribe of Binyamin because of the sin of pilegesh b'givah  was lifted.
3. The nation in the desert ceased dying, and Hashem started speaking to Moshe after a break of thirty-eight years.  In the fortieth year, those who had turned sixty dug their own graves on Tisha b'Av, as had been done the preceding thirty-eight years.  That year, they waited around until the 15th of Av, and they were still alive!  They realized on this day that  the punishment of dying in the midbar was over.
4. The guards established by Yerovam ben Nevat to prevent the ten tribes from going to the Bais HaMikdash were removed.
5. Those murdered in Beitar were given proper burial.
6. They stopped cutting wood for the maaracha, the fire on the alter.  On this date,  sun has already reached its zenith, and the days start cooling off.  Therefore the wood from this day on would not have been dry enough for use.  Also the days start getting shorter, the nights longer,  and it is time to start learning more Torah.

In Megillas Eichah, suddenly, amidst the sorrow and the mourning, we find the pasuk, Kumi roni balaylah, "Arise! Rejoice at night.. "  This refers to the Torah that is learned at night, Torah sheb'al peh.  Even in times of darkness, this is hope and our joy.   This is the joy of Tu b'Av.  This is our atonement.  This is our future.

Let us not leak our emotions out where they do not belong.  It is most unfortunate to allow weakness of character or boredom to lead us to seek  entertainment, amusement and pastime in non-Jewish channels.  As the Shaim miShmuel teaches, joy and fun and merriment are emotions that permeate our entire beings.  We must be most cautious to channel and focus our emotion of simcha on Torah and mitzvos and on permissible pastimes which pose no contradiction to these goals , and rather enhance them.

The dancing and merrymaking around the golden calf extensively delayed the redemption process.
Let us rejoice in Torah and mitzvos, so that we merit, very soon, the words of chazal at the conclusion of maseches Taanis,

In the future days, HaKadosh Baruch Hu will make a machol, a dance for the tzaddikim.  And He will sit in the middle in Gan Eden and they each will point with their finger, as it says, "and he will say on that day, behold, this is our G-d, we hoped and waited for Him and He will save us.  This is Hashem for whom we waited, we will rejoice and be glad in His salvation." (Taanis 31)