(not so) Fun fact: Gusteau is heavily based on Bernard Loiseau, a french chef who published several books and was a frequent guest of cooking TV shows back then. And he pretty much was just a more energic version of Gusteau, very passionate about his craft-.

They ended up almost exactly the same way too. After receiving a very nasty review by a renowned critic, he was on the verge of losing a star on the Guide Michelin. For context, 3 stars is the maximum, and there are only 30 restaurants like that in France right now. Even though he ultimately didn't lost it, he did commit suicide because of this, the stress of losing his relevancy and pretty severe clinical depression.

Being told "You're shit" by someone that everyone believes to have quite the authority on the subject would probably injure most people's ego.
Exponentially so if you dedicated your life's craft to what was called "shit."