Your brain is a problem searching machine, the simpleness of the present moment makes it a place where no problem that instantly may bother you can exist, sure you can worry about some that might happen, but this is future, you can worry about something happened recently but that's the past, and you be annoyed about something that is happening right now, but you are not doing what's needed to alleviate the annoyance, or even more difficultly you are not accepting what's not in your capability to change.

As the brain evolved, it came naturally that chemical reaction is the most efficient way to get the fastest response when in danger, and the same applies when in fantasies.

So whatever you think about will most likely produce the same reaction in your brain that an actual thing that happened produces. Or in other words "the brain doesn't always differentiate between facts and fiction, all happened where the mind was".