Because as you do this work it becomes more and more obvious. Your awareness increases. One minute my son might be telling me about a TV that rolls down into a box, and a few hours later he is telling his Dad about a truck with a back window that slides down. Well if you are not aware, those are two independent conversations; you have so much other stuff going on in your mind that the awareness cannot get through. As you clear out your past and your mind quiets, your awareness increases so that during the second conversation a flash from the first conversation comes into your mind and you realize the second came about because of the first. If you have someone in your life that you do not like, you come to realize that they are a replay of someone similar earlier in your life; you attracted them so you could dig out that old memory and release the charge. It’s an awareness, a “knowing”, that develops as you do this inner work.