The answer to 'why is my water heater constantly running' is that a constantly running water heater is usually caused by a thermostat or heating element issue. This can happen when the thermostat is set too high, the thermostat is malfunctioning, or the heating element has failed and is stuck on.
A constantly running water heater will cause your energy bills to skyrocket, as the heater is continuously working to maintain the set temperature. It can also lead to premature wear and tear on the tank and components. If your water heater is constantly running, it's best to have a professional plumber inspect it as soon as possible to diagnose and fix the underlying issue.