Exhausted after being insulted all day, Zha Ren hid in his nest, growing angrier the more he thought about it. He sat up and cursed: "It even said my blade is just a fire poker, can't even cut people, and should be thrown into the God Star to be burned as firewood."
The Star Descendant wasn't skilled with words, but he repeated the sentence without missing a word, not even pausing, revealing the psychological damage it caused him that he remembered it so well.
Of course, Lightning insulted a lot more, like little stumpy legs, misfit, empty head, dead orphan, but those didn't hurt him, so he didn't remember them. Only the insult about his blade stuck with him.
This Energy Blade was the most unique part of him, even named for it, to be insulted to worthlessness—it was infuriating.
Moreover, Lightning was incredibly cheeky and, seeing the reaction to 'blade is useless,' kept focusing insults on the blade, while provoking: "Come hit me—"