Water heater relief valves leak for 4 reasons.
Please read this first: Relief valve warning

1. The valve is old and it starts to leak, They can just start to leak in less then a year so if it is a few years old, that may be all there is to is so it just needs to be replaced.

2. The water is too hot. water heater relief valves will blow at 210 % f but most of the time the relief valve will blow before that temperature because of the extra pressure caused by the water expanding. This is not to be confused with normal thermal expansion. This is the water heater over heating and is very dangerous. if you have noticed the hot water spitting out of the spout or the water being very hot almost like steam you have a bad control and should shut your water heater off  and have the problem fixed. In this example the relief valve is doing it's job, but may need to be replaced after the control problem is fixed.
3. The pressure is too high relief valves are set at 150 psi. if the pressure coming into your house is 150 psi or more the relief valve will never stop leaking, Now just because your relief valve will not stop leaking does not mean your pressure is 150 psi it may just be bad.
4. Thermal Expansion  When water heats it expands in most cases with a standard 50 gallon 50,000 btu gas water heater with house pressure of about 50 psi it will not expand enough to be a problem. but with higher recoveries and  or higher house pressure, a domestic water expansion tank may be needed to absorb the expansion of the water.

Now that you know what may be the cause. You need to understand that it can be dangerous to mess with a relief valve. You can be scalded or create a very dangerous situation, possibly resulting in death. It is better to have a pro do it for you.

 

If a 40 Gallon water heater springs a leak, How many gallons of water will I have on my floor?

A water heater Will continue to leak until the water is off If you have a water flow of as little as 2 gallons a minute. In 8 Hours (a normal day at work or a normal nights sleep.) you will have 960 Gallons of water on your floor. most homes will get a larger flow of up to 7 Gallons a minute Or more, giving you 3,360 Gallons of water on your floor. IF you took a week vacation 70,560 Gallons of water.

What is the point of this question? Simple, If you have doubts about your water heater, Don't wait till it starts leaking to replace it.