Nobody can literally turn off their brain, that was purely metaphorical.
Everyone is constantly thinking about something, whether it's fast or slow, simple or complex, verbalized or instinctual, purposeful or drifting. That's just the human experience. Even the guys who like to say they "think about nothing" when asked are constantly having some sort of thought process.
That's why people seek entertainment, to direct a never-ending stream of thoughts to pleasant topics, and why just 15 minutes of sensory deprivation can be so painful that a significant number of people would choose to electroshock themselves for fun. Yep, that was a real study.
It's completely normal and natural to be unable to turn off your brain.