There is a tendency, on fast days, for people to concentrate upon the calamities of the past and on the stages that led up to the destruction of the Temple. People reflect on the distant past when they aught - says the Rambam - consider the present.
The number ten symbolizes the peak of completeness and community, the height of sanctity. The destruction of the Temple began on the ninth, but it was on the tenth of Av that it reached its fiery peak. On that day, the Savior of Israel was born.