The most valuable question to ask at this point is, "What's the water pressure in my home?" Water pressure can vary widely and should be one of the first things checked when any plumbing fixture or valve fails. The highest the pressure should be in your house, (Which can be checked with a simple gauge that attaches to a hose bib. $5 at a local hardware store.) is 80 PSI. Pressure higher that this can lead to premature failure of valves and anything else in your house that turns water on and off. (Think faucets, toilets, washing machines, dishwashers, etc...) Some manufactures will even void the warranty on appliances and fixtures if the pressure is to high. If your pressure is to high, call a local plumber and have them install a Pressure Reducing Valve on the main water line running into your home. With this valve you can set the pressure anywhere from 25 to 75 psi.
One other cause of T&P failure on a water heater can be the rising pressure of the heated water in the tank itself. If you have a small or "closed loop" water pipe system, (This is actually created when you put a check valve on your water feed like a Pressure Reducing Valve mentioned above. These valves only let water pass into the house, never out.) when the water heats up in the tank it expands and can create 20 to 30 additional PSI. This is sometimes enough to "pop" the Temperature and Pressure Relief Valve on your water heater. The solution to this (and it is actually required by plumbing code for closed loop systems) is an "Expansion Tank". This is a small exterior tank (3-5 gal) that mounts on the water line going into the water heater and uses an internal rubber bladder to absorb the extra pressure created during the heating of the tank. Temperature and Pressure Relief Valves should still be tested on an annual basis to see if they open and close completely. And it is not uncommon to have to replace them after a few years.
On a final note. If your water heater is not leaking, still provides you with the same amount of hot water you are used to (without turning it up repeatedly), and keeps the temperature of the hot water consistent throughout use, you don't "need" a new water heater. If you are looking to upgrade to more available hot water or a more efficient unit now is probably the time to start looking. The average lifespan for a water heater is about 12 years so you should have a few left. But it always pays to be informed ahead of time. There are many choices out there and you don't want to be forced into a decision during a time of emergency. Good Luck!