The answer to 'what causes water heater' is that there are several common causes for water heater issues. The most frequent problems include sediment buildup, leaks, thermostat or heating element failures, and pressure/temperature relief valve issues.
Sediment buildup in the tank can insulate the water from the heating element, causing the heater to work harder and potentially fail over time. Leaks can develop around fittings, the tank, or the pressure relief valve, leading to water damage and reduced efficiency. Thermostat or heating element malfunctions will prevent the water from heating properly. And a faulty pressure/temperature relief valve can cause dangerous pressure buildup in the tank.