Adaptions

I have a question that I hope will lead to a nice debate

I realize this could go in probably any subforum on this board but I chose since I feel movies are most commonly what we think of when we hear a work is being adapted

my question is this

I've seen a lot of outrage from people concerning adaptions (most recently The Last Airbender and Akira)

I realize with each work comes different complaints (in TLA there were complaints of whitewashing while the Akira adaption is slated to drastically change the setting) but it seems too common that fans of a work will be extremely unhappy with the adaption

I want to know, where does this anger come from?

TL;DR adaptions often include big changes to a work, as a fan why does this make you so angry?