I explained to him that Judaism teaches that all extremes are bad. This is expounded upon at length by the Rambam (in Shemonah Perakim and in Mishneh Torah) who taught that our goal should always be to travel down the path which is center between the two opposites. For example, one who is stingy obviously has a bad character trait. However, the opposite is one who is a spendthrift; and that is equally bad. The proper policy is the happy median; being a cautious spender. The same applies to all characteristics (with one possible exception).

Similarly, Chazal taught (Yerushalmi, Berachos 67a), “Do (service of Hashem) from love and do from fear.” One who serves Hashem with only one of these emotions will not be successful. He has to incorporate both of them into his service and balance them properly.

And so it is with joy and sadness. Neither is good when taken to an extreme. We always must make use of both of them, but in different degrees, depending on the occasion. On Sukkos, the “Time of our Rejoicing,” we read the Megillas Koheles (the Book of Ecclesiastics) which describes a person's demise and the emptiness and waste of materialism. At the Pesach Seder, when we celebrate our freedom from bondage and behave like kings, we wear a white kittel (robe) which reminds us of the shroud of the dead. On Purim, at the height of levity, many sing the prayers with the Yom Kippur tune to remind themselves of the Days of Awe.

Similarly, it would be dangerous to behave for nine consecutive days in total mourning; lest one might succumb, chas veshalom, to total depression. Therefore, the period of mourning begins with Rosh Chodesh, a day of happiness, to balance the sentiments so that we can mourn in the proper measure and not be drawn into a very bad extreme emotion.

May it be Hashem's will that Moshiach come today and all of Israel will be able to eat meat and drink wine even on Tish'ah B'Av itself, Amen.