[BirdieNZ]

Rising tenant incomes. As incomes increase, landlords are able to increase rents proportionally. This is only true where demand outstrips supply, but that's the case in most developed cities in the West, as well as many other cities.

Tenants have several fixed costs to pay: taxes, food, rent, and transportation being the main ones. If their incomes go up, rent typically consumes the increase, because housing is a fairly uncompetitive market.