The point here is that we cannot just stretch muscles if we wish to become flexible. Someone can go to a stretching/yoga class, try real hard, make a little progress, and in four months their net gain in flexibility is small. Why? Their motor nerves are still sending a message to the muscle to stay tight. From learned physical responses, old injuries, bad posture, work habits, emotional stress, the body learned to keep a certain amount of tension in the body.
Only by stretching with the proper mindset can correct this. It is the mind that tells the nerves what to do, which in turn tell the muscles what to do.