The second reason:
The expansion tank on a boiler allows the water in the boiler to expand. Water can not be compressed and when it is heated it expands.
The expansion tank has a diaphragm with 12 pounds of air pressure in it. This allows the the water a place to go when it is expanding. Over time the tank can become water logged or the air leaks out of the tank. when this happens the water has no room to expand, so the pressure builds up and the relief valve will start to leak. the best way to test to see if this is the reason your relief valve is leaking is to watch the pressure gauge when the boiler is heating if the pressure builds while the boiler is heating, the tank is bad and needs to be replaced.