Loss aversion
Losing something hurts more than gaining the same thing feels good. Offer people a sure $500 or a coin flip for $1,000 and most take the sure thing. Offer them a sure loss of $500 or a coin flip to lose $1,000 and most of them gamble, because dodging a loss is worth a risk that chasing a gain is not. That people weigh losses more than gains is one of the best known findings in this field. How much more is still argued over, and later work says the gap is smaller than the famous version and depends on the situation. It is why "don't lose your rate" moves people when "save 20%" describes the same outcome, and it only works on something the buyer already feels is theirs.
The study
Kahneman & Tversky (1979), Prospect Theory: An Analysis of Decision under Risk, Econometrica 47(2), 263-291. doi.org/10.2307/1914185. A sure $500 beat a coin flip for $1,000; a coin flip to lose $1,000 beat a sure loss of $500. That losses weigh more is well established; how much more is debated.
Bose, the headline it swapped
When the Bose Wave system struggled in magazine ads, Bose changed the headline from "New" to "Hear what you've been missing". The product did not change and the price did not change. Robert Cialdini, who says he worked with Bose on it, puts the result at a 45% rise in sales. Bose has published no figure, so that number is Cialdini's and travels with his name on it. The headline change itself is on the record, and the shape of it is the whole principle: the first version offered a gain, the second named a loss the reader was already living with.