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Water heater pressure relief valves, unless defective and needing to be replaced, normally only leak for two reasons.
1. Incoming water pressure is too high for that particular system
2. Internal pressure in the heater tank is too high because of the heating device control (either the gas thermostat or the electric element thermostat) is not shutting off at the proper temperatures.

When this happens (overheating) the internal pressure in the tank goes above the setpoint of the pressure relief valve and causes it to drip or to actually flow water a bit.