Odynophobia is the most common fear in the world. It's the fear of pain.
Most people don't like feeling pain - but being able to feel pain is actually a good thing. Consider the opposite: about 17 people in the United States are born with "congenital insensitivity to pain with anhidrosis" - basically, they feel absolutely no pain. Far from being a wonderful thing, living without pain is actually hell.
Here's a story of (then) 4-year-old Roberto Salazar, who was born without the ability to feel any pain:
When you first meet 4-year-old Roberto Salazar, you can't help but notice his unwavering smile and constant laughter. By all accounts, he's a very happy boy.
It is only when he rams his head violently into walls or plays a little too roughly with a schoolmate, all the while smiling, that you are reminded that he suffers from an incredibly rare genetic disorder. [...]
His family was shocked when Roberto started teething. He gnawed on his own tongue, lips and fingers to the point of mutilation. "If you could imagine when you bite your tongue how bad it hurts. At one point, you couldn't even distinguish that his tongue was his tongue," Stingley-Salazar said.
Doctor Felicia Axelrod of the New York University, who specializes in this rare disease, said:
"For some children it's a mild degree such as breaking a leg, they'll get up and walk on the leg. They feel that something is uncomfortable but they keep on moving," she said. "For other children, the pain loss is so severe that they can injure themselves repetitively and actually mutilate themselves because they don't know when to stop." (Source)