Ode to curiosity

I still remember that I learned to read when I was 4. After that all mayhem broke loose and you could not make me part from a book, regardles. I was so interested in everything, and literally devoured any written material that came to my hands. It took me some time to learn that curiosity was not only about the contents of books, but about what would happen if you kicked a ball a certain way, did something to a frog, or how would people be. During the years I have met some people that are not (or seem not to) be curious about pretty much anything. I really don't understand them, because to be curious is part of what being human is all about (but one we share with other organisms... after all, "curiosity killed the cat", which is a horrible saying). We're all very curious when we're children, but not so much anymore as adults. Why?