Right....density is the reason you're looking for. If more and more people move into an area, it puts more strain on the transit system. Whatever that system is. Cars, trains, buses, whatever it may be it puts pressure.

For trains and busses, this has a secondary affect. Sure, packed roads are annoying because of traffic. But if the traffic increases, which it does no matter what if the population density also increases, the difference is more people have to spend time on the bus/train with more and more other people. If you're in a car, that only manifests in congested roadways, NOT a less pleasent personal commute inside the confines of that transportation.

The worse those shared transportation conditions get, the less people use public transportation and use more cars. Less people, less congested buses, more people take the buses. Really, really basic social consciousness here, my guy.