The part of your brain that was originally designed to keep you safe during times of danger can become confused and misdirected. It can become so befuddled, in fact, that it can start to misidentify safe things as dangerous. We call this anxiety—when you react to and worry about something quite safe as if it is objectively dangerous. When your brain inadvertently reacts to thoughts as if they were dangerous, it sets the stage for unwanted intrusive thoughts to take hold.
