Listen to these sentences:
"The Romans always loved their state."
"Everyone loves mint ice cream."
"All Greeks hated outsiders."
"In teaching his philosophy to others, Socrates never gave answers but only questioned people."
"No one likes getting muddy."
These sentences include overstatement.
"Do not overstate. When you overstate, readers will be instantly on guard, and everything that has preceded your overstatement as well as everything that follows it will be suspect in their minds because they have lost confidence in your judgment or your poise. Overstatement is one of the common faults. A single overstatement, wherever or however it occurs, diminishes the whole, and a single carefree superlative has the power to destroy, for readers, the object of your enthusiasm."
— William Strunk Jr. & E.B. White, The Elements of Style, 1979
In other words:
When you overstate, you accidentally exaggerate what you are trying to say. Your message can get lost. Readers will lose their faith in your message. Even one little overstatement can ruin your piece of writing.
Listen to these sentences:
"The Romans always loved their state." is better as "Many Romans loved their state."
"Everyone loves mint ice cream." is better as "Lots of people love mint ice cream."
"All Greeks hated outsiders." is better as "Many Greeks hated outsiders."
"In teaching his philosophy to others, Socrates never gave answers but only questioned people." is better as "In teaching his philosophy to others, Socrates frequently questioned people. He often avoided giving clear answers."
"No one likes getting muddy." is better as "Most people don't like getting muddy."
Sources:
http://www.usu.edu/markdamen/WritingGuide/03overst.htm
