The fifth reason:
The aquastat or aquastat relay, This is a very important part of your heating system. the item in the picture to the left is a secondary aquastat, it's purpose is to back up the main aquastat, if it fails. Most area codes require this aquastat be installed on your boiler. if you don't have one you should have one installed.
Now if the aquastat fails, the temperature of the boiler can raise to a very dangerous point, causing the relief valve to blow off, this is an extremely dangerous situation and must be corrected immediately, turn the power and gas to the boiler off and call a plumber.