According to Ashkenazic custom, the period beginning on the fast of the 17th of Tamuz and ending after the fast day of the ninth day of Av is the longest slice of time dedicated to remembering any historical event having occurred to the Jewish people on the Jewish calendar.
In the general world such days and commemorations are usually limited to one day such as a Memorial Day commemoration. But to have this period of time of mourning and reflection stretched into weeks is a particularly Jewish phenomenon. One of the reasons that such a considerable period has been set aside for sad remembrance is that mourning and self reflection are processes that build themselves on a cumulative basis.
Our emotions and mindset require time to be able to understand and respond to tragedies, both personal and national. If the fast day of the ninth day of Av would arrive without preparation and introduction, it could very well be deemed only a formality and become an insignificant day on the Jewish calendar. It is the buildup that allows for a true assessment of the events in the history of the day itself.
These three weeks that lead to the commemoration and fast day of the ninth of Av are necessary in order that that special day carry with it significance and historical meaning. Almost two millennia have passed since the events of that day of the destruction of Jerusalem and of the holy Temple in the year 70 CE. The fact that that they have been remembered and commemorated over such a long period of time is testimony to the power of the ritual and observance that this three-week period imposes upon Jewish life.
It is interesting to note that the apparent discomfort that this period imposes upon us is relatively of minor consequence. Even the restrictions regarding eating meat and drinking wine during the days immediately preceding the fast of the ninth of Av are of relatively little discomfort to us. Fish restaurants look forward an entire year to these days. Yet, all the restrictions of the three weeks that precede the fast day of the ninth of Av do have a spiritual and emotional effect upon us, even if only subliminally.
Somehow over the centuries and through the dark and abysmal nights of Jewish history, this time of remembrance kept the memory of Zion and Jerusalem, of the holy Temple and of Jewish sovereignty alive and real. Today's State of Israel is a product of this three-week period. There have been many twists and turns in the Jewish story over the past millennia. However, the one constant has been the fact that the Jewish people instinctively realize that wherever they live in the world and no matter how successful and peaceful their 
Generations now born, 70 years after the founding of the state of Israel and 50 years after the liberation of Jerusalem, really have no background to judge the wonders that have occurred and continue to occur.
sojourn might be, they are not really at home. Home is our ancient land promised to us by Heaven and struggled for by Jews over all of the ages.
There are those who say that since we have been privileged to regain Jewish sovereignty in the land of Israel and that Jerusalem is now a large, modern and inhabited city, there is little reason for us to preserve the observances that the three-week period preceding the fast day of the ninth of Av has imposed upon us. In my opinion this would be a classic example of throwing the baby out with the bathwater. It is only because of the three-week period that we can appreciate the gift that Heaven has bestowed upon our time, in restoring the Jewish people to their homeland and to national sovereignty.
Without perspective, nothing in life can truly be appreciated. Generations now born, 70 years after the founding of the state of Israel and 50 years after the liberation of Jerusalem, really have no background to judge the wonders that have occurred and continue to occur. This three-week period before the fast day of the ninth of Av allows us to frame the events of our time and our current situation. It gives us a sense of gratitude and understanding instead of just relying upon sometimes vapid patriotism and formal staged commemorations. The ninth of Av will yet be a day of joy and feasting when Jewish history has finally completed its long journey.