Some people are curious about what advantages density could possibly have. Higher density puts you in closer proximity to stuff and means that more stuff can be economically supported. That is, having lots of people in an area means that you can have shops, restaurants, supermarkets, etc... in an area without needing acres of parking lot in front of them. Combine a walkable neighborhood with a decent transit system - which itself can be more supportable with higher population density - and you reduce the need for one car per driving age household member as well as removing the primary parental job description of "chauffeur." These things are self-reinforcing. Adding more neighborhood amenities/transit reduces the need to drive, which reduces demand for cars and associated parking space, which reduces space given over to automobiles, etc...
But the density needs to be combined with other things for it to work. You need mixed-use zoning in some areas at least so people are mixed in with retail/jobs. You need calmer traffic areas so that being a pedestrian isn't a health hazard. You need a sensible, if not perfect, mass transit system.
