Why do people get anxious about nothing?

So I'm talking about my father. He never says it out loud, but it's obvious that he gets anxious about the most trivial things. He doesn't like to cut into a different lane while driving to be earlier. He doesnt like to go out in general especially if there are a lot of people out. He doesn't like to go o vacations to other countries. He hates asking people to do things for him because he doesn't want to bother them. It's all those litle things like that which add up and make him seem anxious about that stuff. I just don't get it. Why does he get anxious over such miniscule things?