Yeah, that's the 'receipts for everything' idea and if you think it through, you'll realize it's at a minimum impractical and more likely just an outright bad idea. Where are these 'reasons' going to go? Who is going to read them or act on them? It sort of wants to stick it to those bad flaggers and misinformed downvoters or whatever but think about it applied to you. Do you not recoil at being asked by some random web app to justify your actions? Like, we're ostensibly here for conversation not to fill out TPS report cover sheets.

This is 'drink verification can' but for messageboards.

A lot of HN mechanism makes more sense if you can accept the idea that the goal is to promote good threads, and not, as so many people believe, to promote one set of opinions over another. Requiring justification for flags would immediately crud up threads with meta-debates.

A hard thing for people to accept, something that I think is an unstated part of the HN ethos but nevertheless real, is that it's almost always better to have no thread at all than a shitty one. Important topics will inevitably get an airing in one thread or another.