Just like your tank, the fittings on the T&P valve can get coated with calcium and whatever else is depositing out in your tank. Then, when you open it, you scrape off some of that on the stem, but it sticks a little, not reseating. If there is a large accumulation, it's worse. You can try to remember to open it on a regular basis, and that will help, but you'll still end up replacing it occasionally. At least then, though, you'd know if it was too gummed up to operate and unsafe.
Sounds like a water softener might be a good addition.