Consciously or subconsciously, we allow thoughts to linger in our minds, we entertain them, and make more thoughts of the same kind, until this becomes a belief, and we have no longer control over it. It is now in the subconscious mind, and it will run the show in the background and dictate from now on what thoughts you will have and actions you will take. So the basis of Mindfulness is exactly this - start observing, because each attempt of observation is actually a torch in your hand, shedding light to the direction you are observing. Have fun and enjoy!