Can you listen to music in the background?

I listen to music literally 24/7. When I'm sleeping, when I'm reading, when I'm watching TV, when I'm eating, when I'm doing homework. But I hear it and then again I'm not hearing it? The music is just there and I can focus on it at the same time as my homework or book if I wanted. Then other people will insist on silence when doing work, but why?