Editor’s note: Dr. Charles Raison, CNNHealth’s mental health expert, is an associate professor of psychiatry at the University of Arizona in Tucson.

Humans evolved in a dangerous world.

Because of this, our brains and bodies are wired to be overly sensitive to threats. This phenomenon is often referred to as the “smoke alarm” principle, based on the idea that it is better to be awakened 10 times in the middle of the night by a false alarm than to sleep blissfully just once while your house burns down around you.

BOSTON—The nurse who tended to Boston Marathon bombing victim Krystle Campbell near the finish line during her final moments of life would like to meet her parents in the hopes he could give them some solace in their grief.

Stephen Segatore would like to tell Campbell’s parents that she didn’t die alone, and she didn’t suffer for long.

When EMTs carried a woman into the medical tent where Segatore was volunteering, he was struck by her beauty, her youth and her bright blue eye shadow.