Your brain won't remember or deeply comprehend things it doesn't care about. It won't waste the energy.

This is why it's important to align yourself with what you care about, it's the only way to be as smart as you have the ability to be. Which is hard to do because it requires peeling back the onion layers of your 'memetic' desires (desires you somehow picked up that you don't really care about).

This is also why our largely sterile schooling process sucks.

(Don't actually know if that's what Plato meant here)