When the student is ready, he sees the teacher.Unlike another common phenomena where you learn a new word and then start hearing it everywhere or meet a new person and start seeing them everywhere, this isn't so much about noticing. It's about projecting*. Unconsciously.
Let's say you are madly in love with a lice-infested, unemployable, ill-tempered, moose of a man. You wonder whether you should stay together though, in your heart of hearts, you are completely confident in your feelings. Everything you hear on the radio points to yes! If the song is about breaking up, you notice that the guy being broken up with sounds nothing like your catch. If the song is about staying together, you notice that the song is about love and commitment and perhaps the guy in the song who is so worth committing to sounds a little like your lousey love interest.
Now instead, let's say your heart of hearts was actually singing a different tune. A song about breaking up would instead be a message, "You should break up!" A song about staying together will help you realize "this is the kind of relationship you deserve and you don't have a shot at it until you drop the loser you're with!"
The words of the song haven't changed. But what you are able to read out of them has. When you are ready to know something, you will begin to run into people / books / road signs that can teach it to you. However, depending on how coy your subconscious self is (or how inattentive your conscious self can be) you may not notice half the time.
At the Museum of Science in Boston a few years ago, I came across a vase on which an optical illusion was painted. If you looked at it one way you would see dolphins frolicking in the ocean. If you looked at it another way, you could see two lovers embracing. (Click here to see an example of this kind of illusion.) The display description that went with the vase mentioned that most adults could easily see both images. It also made the following assertion, which really captured my attention: an innocent child would not / could not see the embracing couple.
What it is that we cannot yet perceive, and therefore learn from, because we are not yet ready?
A world full of teachers surround us. To those we cannot yet see, please be patient. In one way or another, we are innocent children.
*Unfortunately, perhaps due to self-help jargon abuse, the word "projecting" has a bit of a negative connotation. I believe the negative aspects of "projecting" are a result of the student being ready to learn something that is completely incorrect.