Code of Jewish Law says: "All who are in awe of G-d should feel pain and the anxiety over the destruction of the Temple." (OC 1:3)
Upon reading this, we may feel "pain and anxiety" over not observing this mitzvah properly! But don't worry (too much that is), because what we'll try to sort out here is the meaning of this interesting law, and the way to practice it effectively.

How is it possible to live in pain and worry and live a normal life? Furthermore, pain and worry lead to depression which makes a person so tired and weary it may become impossible to do the mitzvahs!
The first thing to understand is
that Judaism does have a general obligation to be happy - "simcha!" (see Mishnah Berurah 3:9)
However, there is a limit. Even
at the time of our greatest joys, there is a requirement to "remember the
destruction of the Temple." (OC 560)
The Talmud (Brachot 31a) relates that
at the wedding of Rav Ashi's son, the guests were having "too much fun."
Rav Ashi took a beautiful cup of white crystal and smashed it on the
ground in front of them. Immediately everyone became sad. The Marhashah
(17th century, Poland) explains: The crystal glass reminded them that just
as crystal is formed out of sand, but is easily broken, so too was man
made into a beautiful person, but eventually he will return to dust.
The actions of Rav Ashi's are
mirrored today at weddings, as it is customary for the groom to break a
glass by stomping on it, in remembrance of the destruction of the Temple.
(Tosafot, Rama) Additionally, ashes are placed on the groom's forehead
where Tefillin is placed.
The great Rabbi Yaakov Yosef
Herman, the subject of the book "All for the Boss," put cards on each
table at the wedding of his daughter announcing that dessert would not be
served because of the destruction of the Temple. This custom of not
serving a full meal is mentioned in the Code of Jewish Law 560:2. It
applies to all meals except Shabbat and Yom Tov. However, the Mishnah
Berurah says it does not necessarily mean you need to miss dessert; the
desired effect can be achieved by not serving even an appetizer.
Jewish tradition prescribes other
remembrances of the Temple's destruction. For example, there is a custom
of leaving a patch of a wall unpainted, roughly 18 inches by 18 inches,
opposite the entrance to a house.

EXPERIENCING SORROW
The Talmud (Ketubot 62) relates
the following story:
An Israelite and an idolater were once walking on the same road and the idolater could not keep pace with the Israelite. So the idolater mentioned the Destruction of the Temple, causing the Israelite to grow faint and slow down.
The idolaters knew that if you
ever needed to slow a Jew down, just mention the destruction of the Temple
and it would depress him enough to do the trick.
Today, how should we fulfill the
obligation to be "pained and worried" over the destruction?
The Mishnah Berurah (1:3) gives a
few practical suggestions:
- 1) Wake up at midnight every
night to say "Tikkun Chatzot," a collection of Psalms. If you check out
the Western Wall in the middle of the night, you will see people praying
the "Tikkun Chatzot."
- 2) Before saying Grace After
Meals, recite Psalm 137 - a lamentation about the exile of the Jews.
At the very least, a minute or
two should be spent everyday thinking about the Temple, and the great
spiritual opportunity that was destroyed with it.

"TEARING KRIYAH"
There is a mitzvah upon seeing
the site of the Temple (not only the Western Wall but also the whole
Temple Mount) to rip ones garment, as a mourner does for a beloved one at
a funeral. (Code of Jewish Law 561) This is called "tearing kriyah."
This rip, however, is not simply
a mechanical act of tearing your clothes. Rather, tearing kriyah should
help you to feel an emotional rift in the depths of your heart, as it is
written: "Rend your hearts and not your garments" (Yoel 2.13).

A POSITIVE NOTE
Rabbi Yochanan, the great
Talmudic scholar, said:
"During these times that the Temple is demolished, a person
is not allowed to fill his mouth with laughter. This is because the
verse (Psalms 126) says, 'THEN our mouths will be filled with laughter,'
and does not say 'NOW our mouths will be filled with laughter.'
"And when is 'then?' 'Then'
will be when the Third Temple is rebuilt."
- Brachot 31a and Code of Jewish Law 560:5
This means that the mitzvah of being "pained and worried" over the destruction of the Temple is a temporary mitzvah. It only lasts as long as we don't have the Temple! When the Third Temple is
built, THEN we will fill our mouths with laughter, THEN no glasses will be
broken on wedding days, THEN no meals will be left incomplete.
And who knows, perhaps THEN the
Jews will be laughing over another irony: Tisha B'Av - the most tragic day
for the Jewish people, the day of the destruction of the Temple - is also
the birthday of the Messiah! (Pesikta Rabbati 29)