Higher densities of people make many things more efficient, e.g. shared heating in apartment building, making mass transit viable, sharing resources like food and water. Climate change is forcing our hand on transitioning to a higher-efficiency society, but we would have to do it soon anyway because we are running out of oil. When you can't enjoy the energy density of fossil fuels, efficiency will start to matter a whole lot more. Our current mode where a single person might travel up to 50 or 100 miles every day, just to live their life, is a ridiculously inefficient way of spending your time on Earth.
Secondly, habitat loss is the single greatest cause of the mass extinction of species we are currently experiencing. Most of that is agriculture, but urban sprawl certainly plays a big part. If we want to enjoy, you know, other species existing, we should be looking for ways to reduce the amount of land we need to occupy as humans.
Also, a bonus: kids that grow up in walkable neighborhoods have been shown to have the highest levels of upward economic mobility. So really if you want to do right by your kids, you should be moving to a walkable area and reducing your car use.