Since birth our brains are wired to look for faces. It helps with survival when the helpless wiggly thing bonds with the giant who is full of hormones telling them to protect it.

As we grow we learn to recognize other patterns, which help us find food, be safe, find a mate, etc. Our brains are constantly looking to match everything we see with something from a previous experience. Which is unfortunately one of the places PTSD can pop up. Say you had a traumatic experience - you may not remember seeing someone wearing a red hat just prior to something terrible happening, but your brain might. In the aftermath it's possible that you find yourself uncomfortable around someone wearing a red hat but can't figure out why. You may not remember, but your brain does and thinks it's helping by alerting you too a problem.