The period of time between Shiva Asar B'Tammuz and Tisha B'Av is known at "Bein HaMetzarim", a phrase that appears towards the beginning of Megillas Eichah which literally means "between the narrow straits", but is understood by the Midrash to mean "during the days of distress". These three weeks are undoubtedly the saddest on the Jewish calendar and it is our practice to mark them by various observances associated with Aveilus which increase in terms of intensity as the weeks progress, culminating of course with Tisha B'Av itself.
It is important to bear in mind here, as is often the case, that the practices mandated by Halacha and Minhag are designed in part to create a certain atmosphere, a special mindset, that will enhance our appreciation of what these days are all about. Many of us are familiar with the list of varous tragedies, preserved by the Mishnah in Ta'anis, which befell the Jewish People during this time of year. By exhibiting feelings of Aveilus, we are declaring, to ourselves and to others, that these are not obscure, irrelevant events that took place long ago and no longer impact us, but rather are part of our national psyche. Recalling them allows us to focus upon our relationship with HaShem, to explore what caused these terrible events, and to examine ways to do Teshuvah and rectify things for the future.
Along these line, it may be noted that in the future, these days will, G_d willing, become days of great joy and celebration. The B'nai Yissoschor points out that the parshiyos of the Torah read during these three weeks, namely Pinchas , Mattos-Masei, and Devarim, all discuss issues relating to the conquest and division of Eretz Yisrael. They thus call our attention to our connection to Eretz Yisrael which at once causes us to mourn what we no longer have there, but at the same time to rejoice with the knowledge that HaShem has promised that there will indeed be another, more permanent settling of Eretz Yisrael by the Jewish People.
May the reading of these Parshiyos about Eretz Yisrael bring us some comfort and may we all merit seeing the transformation of these three weeks to a time of happiness B'Meheirah Veyameinu.