The Rambam, in his commentary on the Mishna (tractate Rosh Hashana: 1,3), states that during the period of the Second Temple, there was fasting on Tisha B'Av:
"In the days of the Second Temple they did not fast on the tenth of Tevet, nor on the seventeenth of Tamuz; but rather whoever wished to fast – fasted, and whoever did not choose to – refrained from fasting. Therefore they (meaning the emissaries for the testimony concerning Rosh Chodesh) were not sent out on Tevet and Tamuz. 'So said the Lord of Hosts: The fast of the fourth (month) and the fast of the fifth (month) and the fast of the seventh (month) and the fast of the tenth (month) etc.' (Zecharia 8,19) – thus leaving the choice to them regarding these days: to fast or not. The fast of the fourth is Shiv'a Asar B'Tamuz, Tamuz being the fourth month, and the fast of the fifth is Tish'a B'Av – Av being the fifth month, and the fast of the seventh is Tzom Gedaliah, and the fast of the tenth – Asara B'Tevet, Tevet being the tenth month. And they did fast on Tish'a B'Av despite being given a choice, since on this date our people suffered multiple tragedies."
The Tashbetz in his Responsa (part 2, 271) wrote that clearly the law of fasting is in effect today too, but was not so during the period of the (second) Temple.
The Rambam's opinion seems odd. How is it possible that people fasted when the Temple was intact?
The simple explanation seems to be that the second Temple was not complete. The returning exiles themselves wept because of this. The second Temple lacked two vital elements - the Ark, and possibly the Divine Revelation too. That is the reason for fasting for the destruction of the first Temple. Let us try to find additional explanations.
The S'fat Emet (on the Talmud, tractate Rosh Hashana 18B) is of the opinion that when the Temple was intact - in times of peace they did not fast even on Tish'a B'Av. On the other hand, during periods of turbulence and subjugation to other nations – they did fast.
However, the straightforward interpretation of the Rambam's words is that they always fasted on Tish'a B'Av. In any case, it is interesting that the Talmud says that the relevant factor in observing or canceling a fast is not the existence of the Temple, but rather the state of national affairs: Is there peace or not?
We can infer that we fast not only because of the past; the fast is also due to the present situation. In addition, it serves as a reminder for the future. Since it relates to the current state of affairs, it is contingent on peace or war in our times. Our present troubles are an extension of the destruction of the Temple. This is why the Kinot (lamentations) on Tish'a B'Av include many events not directly connected to the destruction, but rather - other troubles in the course of our history – such as the persecutions 1096, the burning of the Talmud, the Holocaust and others.
Secondly, the fast reminds us that even when everything seems fine, and the Temple is built, we must live righteously; the situation can, G-d forbid, change.
There may be more to this. Sometimes we have it so good, that we fail to recognize our good fortune. Maybe fasting even in good times is significant because it reminds us of periods in the past when we were not so fortunate, and lacked what we have now. Sometimes, one can see and appreciate the light only when contrasted with darkness. Only by reflecting upon the destruction can we truly perceive the supreme and wondrous reality of the Divine Presence.
This may be the special meaning of the Tish'a B'Av kinah "Eish Tukad B'kirbi" (A fire shall blaze within me) which enumerates the wonderful things that we had when the Temple stood, which are missing today (similar to what we say on Yom Kippur immediately following the service of the Kohen Gadol).
Sadly, today the Temple remains in its destruction. True, Hashem has granted us many kindnesses. We have been fortunate enough to return to our homeland after two thousand years. We have been granted a state of our own and have seen part of Yerushalayim returned to us. We see the Torah flourishing. However – many elements are still missing. There is still a spiritual deficiency; we do not yet have full control of our land; but mainly we do not yet experience the Shechina (Divine Presence) among us and Hashem's rule over us.
On Tish'a B'Av we weep for what we are lacking, for the destruction of our Temple, for the departure of the Shechina. While weeping we remember additional events which derive from this destruction – such as the expulsion from Gush Katif and the uprooting of its residents. Yes, we also remember all we have been given, thank G-d, and thus the weeping takes on an added dimension: Do we really behave in a fashion appropriate for people who have received so much?
Moreover, since we have achieved so much, if we have seen the beginning of our redemption – we must have the ability to reach ever higher. The higher we rise, the more we enjoy the gift of freedom - the more concern we must show for those who are less fortunate and suffer hardship - and help them, too, fulfill their great dreams. Baruch Hashem, we have been granted the privilege of assisting many people - including Ethiopian Jews, lone soldiers, at-risk youth and others. Showing solicitude for these groups is by no means for their benefit alone. By elevating their lives and enabling them to aspire to greater heights - all of Am Yisrael will find itself on a higher plane of existence, and will, please G-d, soon merit complete redemption!