I think just having more people makes the area more lively. Less people will basically turn it into suburbs. Also higher density means public transportation can

I think just having more people makes the area more lively. Less people will basically turn it into suburbs. Also higher density means public transportation can be better utilized and reduce the need for cars. I saw this at Arlington and Nova where right around the metro stations, there were high-rises and the area was very walkable and lively.