If everybody liked the same things to exactly the same degree, there would be no motivation to create new things - all music would sound the same, all art would look virtually identical, all stories would be as predictable as romance novels
, and the world would become infinitely bo-rinng...
Not to mention we'd run completely out of all the stuff everybody liked.
Or are you asking why no matter how well you do something, there's always somebody who can't do anything but criticize and complain? Because they're @ssholes.
Seriously, some people really do thrive on drama and negativity. But if we didn't have those jerks to measure by, we might not appreciate it when we met a genuine sweetheart.