There are a couple of things that may be causing this. The first thing is that the pressure in your home may be too high. If you have a pressure regulator located on your main line above the shut-off valve, it may not be working anymore. Also you may want to check the temperature of the water in your water heater by checking the gas valve knob on the bottom of the heater. If it is turned all the way hot, the water is too hot. This will also cause the valve to leak. Other possibilities could include the valve has just worn out, but that is more rare. To check the pressure in your home, you can purchase pressure gauges that screw onto your hose bib outside. Turn the water on and see what it reads. Normal pressure should be anywhere from 50 – 75 psi.