This is such a great principle, just observing what you are actually doing/feeling. But you need to be in the right mindset or trained to do it.

Pain is a good example: If you begin to observe it as a simple reaction by your body, not as a state of yourself, it can become very bearable. If the pain is too strong however or you don't have the relaxation it takes to do this it doesn't work. To become an observer is hard with strong headache on a busy day, first you have to remember to become an observer and than it still might overpower your state of mind.