The entire Crownsguard kind of treats Gladio like shit because he's better than them at most things, having been trained since he was a kid, and he also was sort of born into being Crownsguard, so they feel like he didn't earn it. By mid-to-late teens, Gladio's learned to put up with it.

Then Ignis joins up and starts to hear how they talk about him.

+ Ignis was only really ambivalent about Gladio until now, but starts to realize he and Gladio have a lot in common
++ Ignis trying to go with it to fit in at first but losing patience
++++ Ignis and Gladio bonding, and Ignis trying to make Gladio feel better about it, because even though Gladio says he's fine, Ignis is pretty incredulous about that