Why do some people worry more than others?

I am a worry wart. I have the tendency to think the worst thing must have or will happen. If a situation isn’t playing out like I think it should, my mind starts overworking and I think up crazy scenarios.
For example, if my grandma doesn’t answer her phone or my boyfriend forgets to call and let me know he’s home safely..oh my something terrible has happened!
Why do some people worry more, while some are laid back?
Do you tend to be a worry wart?
How do you put your mind at ease?