Reply to Learning/Practice Theory, why do results become harder to achieve over time?

I'm sure most of you have heard this theory or even experienced it yourself: When you first start something, the learning is very rapid and very rewarding, but as time goes on it seems that learning and achievement takes longer and there are more gaps between rewarding moments.

In theory, wouldn't you believe the opposite should be true. Starting out should be very difficult and be the least rewarding, but as you acquire more knowledge and skills it should become easier to learn and discover new things (rewards). What do you guys believe?