Why is a grilled lamb chop tougher than stir fried beef strips or lamb stew?
There are many variables that determine the final texture of cooked meat.  Some of them include how and what the animal was fed, which muscle and how much fat and collagen are present, treatment before cooking, cooking method, length of cooking time and temperature.  The list goes on.  Let's just take your chop vs. stew example.  A lamb chop is relatively lean and cooks quickly.  Stew meat, taken from a different muscle, is relatively fatty and has collagen that is broken down by a slow and lower temperature cook.  The latter is usually more tender because of those variables. (Also see your other question about stir fry).