It's just one of those things, like Goku seeing Krillin get murked, there's no practical reason but it serves as a trigger for the character to tap into that hidden potential. Orihime's silly ass is a catalyst for many things and considering her broken ass power, she should be capable of more. Doesn't hurt that she's the most important person to him.
Grimmjow did the damn thing though, man went out of his way to shoot the fair one with Ichigo. Literally saved him from the brink of death, brought his girl to witness it, trapped Ulquiorra, let Ichigo power up to the maximum, and still whooped his ass in style. Grimmjow been doing shyt his way since before the Espada and didn't switch up.
... and here you go watching 21 episodes of the Amagi arc filler. You a soldier, I'll give you that.