1) In big cities, you are anonymous and can get lost in the crowd easily. Each day you see a new face. Relationships are transient.
2) Air, noise and light pollution. You can hardly get proper rest when exposed to pollution. It has a cumulative impact on health.
3) The fast pace of life wears on you. The constant hustle and bustle of cities distracts people from the natural beauty around us.
4) People who live in dense cities tend to be a lot more sensitive, this is due to the high stress of dense urban living.