Why do I grind in the valves?

Here's some better close-ups of the valves before and after:

As the valves get pitted they loose efficiency (let gasses past when they shouldn't)

This is an exhaust valve - you can see what 140,000 miles of opening and closing on hot gasses has done to it. This is probably the worst one in the engine.



Here's it's valve seat - same sort of pitting.



After some grinding this is the valve (it looks worse than it is, it's quite clean when you look at it!)


Here the ground seat on the bottom and an unground one on top.