Once you have learnt something you see the world differently. Not only can you appreciate or do something that you couldn't appreciate or do before, but the way you saw the world before is now lost to you. This works for the small things as well as the big picture. If you learn the meaning of a new word, you won't be able to ignore it like you did previously. If you learn how to make a cup of out of clay you won't ever be able to see cups like you used to before.
This means it is hard to imagine what it is like for someone else who hasn't learnt what you've learnt. The psychologist Paul Bloom calls this the curse of knowledge in the context of being unable to model what other people don't know, rather than on what you yourself used not to know. If you've ever organised a surprise party for someone, or had another kind of secret, you'll know the feeling. It seems so *obvious* what you are keeping hidden, but usually the person you are hiding it from doesn't catch on. They don't catch on because the clues are only obvious to you, knowing the secret, and you find it hard to imagine what they see not knowing it.
The reason this occurs is because of two facts about the mind that are not widely appreciated. The first is that memory is not kept in a separate store away from the rest of the mind's functions. Although there are brain regions crucial to memory, the memories themselves are not stored separately from the regions which do perception, processing and output. Unlike a digital computer, your mind does not have to fetch stored information when it needs it, instead your memories affect every part of your perception and behaviour.
The second important fact about the mind is related to the first. It is that learning something involves changing the structures of the mind that are involved in perception and behaviour. Memories are not kept in a separate store, but are constituted by the connections between the neurons in your brain. This means that when you learn something --- when you create new memories --- it isn't just *added* to your mind, but it changes the structures that make up your mind so that your perceptions, behaviour and potentially all of your previous memories are changed too.
We can see this in microcosm if we look at a small example of what is called one-shot perceptual learning. What do you think this is?
Now probably you don't know, but I would like you do savour the feeling of not knowing. Try and taste, like a rare wine, what the perceptual experience is like. You can see the parts of the picture, the blacks and the whites, various shapes, some connected to others and some isolated.
If you now look at this popup here then you will have this taste washed out of your mind and irrevocably removed. It will be gone, and you will never be able to recover it. This is why I asked you to savour it. Now look at the original again. Notice how the parts are now joined in a whole. You just cannot see the splotches of black and white, the groups, the isolated parts, again. When you learn the meaning of the whole picture this removed the potential for that experience. Even the memory is tantalisingly out of reach. You can't recover an experience that you yourself had two minutes ago!
One-shot learning is unusual. Most learning happens over a far longer time-scale, so it is even harder to keep a grip on what it was like to not know. All of us will have had the experience of a bad teacher who simply couldn't see why we had a problem - they simply couldn't see that we couldn't understand or do what was obvious or easy to them. A good teacher has to have the dual-mind of knowing something, but also being able to empathise with someone who doesn't know it, someone for whom what is obvious isn't obvious yet. It is because learning has this tendency to make itself invisible that teaching is such a difficult and noble tradition.
Link: A Mindhacks.com post in which I discuss a similar thing in the context of the role expectations play in our perception.
The reference I took the picture from: Rubin, N., Nakayama, K. and Shapley, R. (2002), The role of insight in perceptual learning: evidence from illusory contour perception. In: Perceptual Learning, Fahle, M. and Poggio, T. (Eds.), MIT Press.