Population density is more people living in a given area.  In order to have increased population density beyond what a block full of houses can hold, buildings are built taller.  In order to sustain these tall buildings, rapid mass transit is needed.

Why is population density desirable? Higher density areas allow for better use of land in both the city and the countryside, it creates a more lively city with more shops, restaurants, and services within walking distance. Higher population density also reduces automobile usage, thus decreasing our dependence on foreign oil while improving air quality. Density gives an area vitality.

City density is the best friend of the countryside. Woods and farmland can be preserved and used for their what they do best, rather than eliminating them to build more pseudo-city (suburbs).

There are people who claim that market forces should control density. But if market forces had been allowed to prevail in the first place, then many suburbs would never have come to be and others would never have grown. State and Federal governments have paid and continue pay for roads, electrical grids, water, sewers, and other government supplied services.  If government was not paying to install the roads and utilities, much suburban development would not have taken place. Cities have subsidized building suburbs, even while the same government entities have refused to help pay to updates utilities that are already in place.  Fiscally, this is irresponsible and gets even worse; the more miles that these services are spread across, the more miles of utility are needed. With the same number of people, it actually costs more per person not only to add this infrastructure, but also to maintain it.