The third reason:
The pressure reducing or fill valve for a boiler is set to only allow 12 psi in to the boiler.
If this valve fails it will allow the pressure in the boiler to reach 30 psi or higher. causing the relief valve to leak. if your gauge goes over 30 psi and the relief valve does not leak, turn the boiler off and call a plumber ASAP. This is a very dangerous situation. To see if this is the cause of your boiler relief valve leaking, please use the 24/7 online help.