Because in their mind a certain noise or memory can trigger the horrible events they have gone through. When he plays "I'll cover my eyes" it's like saying they take the pills they are given, to help them forget the terrible memories. Who choose to close their eyes, because they have the opportunity to open their eyes but this means facing their fears head on. The next lines “for if the darkness returns, my brother will die” fits perfectly into the mindset I am creating. Because for them, stopping taking drugs means that they will have to open up and give light to the oppressed memories that torment them at night. They will be forced to recreate and experience the moments that caused them so much pain and suffering. When he says "my brother will die" he tries to show the audience that it doesn't matter if the event happened 10 years or 5 months ago. For them, the death of their friend or brother will be relived as if they were dying right now in front of them. Causing them to experience each emotion again and again until they can numb the emotions by taking the