Because when everyone is only nice, you wonder what could be improved, and sometimes when people come up with questions that point out weaknesses, that makes for interesting new ideas. (Not that I don't like the nice comments. I roll around in those like a happy cat in catnip, like most people do. )
In practice though, when people make criticism, specially lengthy ones my reaction is less: ooh, good point, and very often more : ... but... that idea you hate... I... don't think that's what I wrote? Or at least, it wasn't what I meant to write...?
The weirdest thing about writing fiction is the things the readers make of what you wrote.
I didn't intend to write Elrond as uncritical of Maedhros and Maglor. Or Elros, for that matter. I intended them to present their opinions directly to Maedhros and Maglor, and to show concern and suspicion from Elrond's point of view, but never voice it to anyone else.
I didn't intend to write a Maglor who was an incompetent disaster that needed rescuing by Elrond, either. Or malignant Valar, or Elwing, Indis and Ingwion as stupid or horrible....
Perhaps the writer is the only person who actually reads the story as they intended it to be, and everyone else makes their own story from the jigsaw pieces they present.
Maybe there's a lot of communication that is like one of those conversations in a loud place where neither of you can quite hear the other one so there's a lot of nodding and smiling and hoping you didn't just agree to anything too outrageous.