Because you are the only one who can evaluate your own reality, if the senses you use to perceive that reality slowly and extremely gradually fade away then you would not notice it. Of course, this begs the question of whether your senses are the same as they were a year ago, or a month ago, or a week ago. Really, every time you go to sleep (or fall unconcious) and then reawaken, you could be seeing things a totally different way. But most of us are so consumed with life that we don't bother to even consider it and continue with the day.