Where does the time go?

I have a 6 month old son, Troy, who is the absolute love of my life. I'm lucky enough to get to stay home with him, all day. Sometimes, I just watch him. Just watch him experience life. He LOVES it. He just loves everything about life. Water bottles, blue yarn, the sound of a duck. It alls makes him ecstatic! Literally laugh out loud happy. When does that go away? When in life does that happiness fade away? When do we stop loving everything and taking it for what it is? I mean, if I could keep him in this stage of his life forever, I would, not to keep him as a baby forever so that he could never move out of the house or anything ... but so that he never has to experience what the harshness of the real world is like. So that he never has to lose that light in him that makes him appreciate a water bottle. Seeing the light in a child is the best thing in the world. I would hate to watch that slowly fade from the life I took 9 1/2 months to make.