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 test for this problem, turn the boiler off, wait till the boiler is cold, drain off some water till the pressure drops below 10 psi, then with out turning it back on, wait to see if the gauge starts to go up again if this happens it is probably the fill valve (it may also be Reason Four) to eliminate the fifth reason turn the water to the boiler off, if the pressure does not raise with the water off, it's the fill valve. if it still raises with the water off, it is reason four.