If you've ever tried teaching an unteachable person, you already understand.
They come to you for instruction, but resist every word you say. They close themselves off from anything new. They already 'know it all'. Of course, the one that misses out is them. They sabotage their own opportunity to go to a new level of understanding.
The thing is, we have all been there at some point.
Especially, as we get older, it can get harder to stay teachable. Hence the phrase, "It's hard to teach an old dog new tricks." We can get set in our ways. In our thinking. In our 'this is how it is and no one can tell me otherwise' mentality. And, the one that misses out is us.
It's like a man that lives on an island his whole life. He knows that island like the back of his hand. And, He truly believes that nothing else exists beyond that island. No one can tell him otherwise.
A wise man knows that he has not yet arrived. He realizes he would be a fool to think he knows it all and to resist anything new. He is careful to stay child-like and open to his Teacher's Words. He never stops being teachable.