Our brains only process a small amount of the sounds feeding into them. The motor on your refrigerator, the constant bubbling of the fish tank, and the soft drone of the A/C unit get cut out. We don't hear them unless we take a moment to think about them and make a specific point of hearing them. I think it's so that we don't go insane. I would argue that this is the same reason some us are so good at toning out select co-workers so completely.