Why Do Some People Prefer Bitter Drinks?

image credit: Dinah Sanders cc
There's been a wave of popularity for drinks like the Aperol spritz, the Negroni, and a host of cocktails flavoured with 'bitters.' There is now a definite trend towards bitter drinks.
People are ordering whisky or gin-based drinks paired with vermouths. And there is growing interest in the US, UK and other European nations in Italian amari. Why are people turning their backs on sweet cocktails in favour of a bitter taste?