Normally our brains tune out a bunch of information that it doesn't feel is useful to us. Right now, there are probably sounds that are coming to your ear, that you aren't aware of because you attention is focused on other things. The sound still gets processed, your mind just filters it out. Worry about danger is soemthing similar. You could focus on everything that could hurt you. Driving in cars is statistically very dangerous. Tiny meteors kill people, spiders kill people, people die of food poisoning, bacteria, etc. If you wanted to stay alive, you could focus on everything that is a potential threat, but then you wouldn't really be able to function. So your brain ignores a bunch of that stuff. People who are hypochondriacs are not able to filter out as much as you do. Their brains tell them they should be concerned about more of that kind of thing.
