Watching my son learn how to walk, talk and do various things around the house is so fulfilling and also enlightening. One reason why kids learn fast, I think, is because they forget fast. Kids have a good short term memory but a poor long term memory. Thus they can continually focus on things they are doing right and what they like, while forgetting all they did wrong and disliked. Thus repetition creates a long term memory which is right.
On the contrary, adults tend to have a better long term memory and a poor short term memory. I think the short term memory does not deteriorate with age, rather the long term memory overtakes it. We remember not only the things that we did right and that we liked but also the things that we did not like or did wrong, which affect the right short term memory to be repeated right, which affects our ability to learn the way kids do. When we delve on the past, we give up the present hence forming a future that is in lines with the past and the repetition hardens us a person, our ability to learn, to think.