Ironhide didn't understand. He just didn't.
Humans were such a primitive, violent race. They feared everything; they practically lived off of fear. Every time something came along that seemed powerful or was just something they hadn't seen before, they do not even attempt to negotiate. The humans attack it with all their strength. They didn't want anything to do with something they had never seen. They just wanted to weaken or destroy it so they wouldn't have to know anything about it.
It had happened with Bumblebee. The poor soldier hadn't even fought back. And yet, the humans kept attacking him, freezing him, shocking him. He could've been killed easily because of their fear and anxiety. Humans could've destroyed him. They didn't hear his cries. They didn't listen. They didn't think for one fucking minute that they were there to help. They just wanted to kill Bumblebee and make him an experiment so they could find out how he worked and figure out what it took to make sure he and the rest of them wouldn't come back. It was all fear.
Ironhide just couldn't see what Optimus saw in the humans.
It wasn't until after the battle in Mission City that he truly saw the strength and defiance humans displayed in the face of peril and danger they could not possibly have prepared for. They lived and they protected one another, risking their lives like one family of brothers. They were just like Ironhide and the others. They shielded one another. They didn't back down. They died for one another.
The humans were just like them. Not as smart, nor did they have as broad of minds, but they were just like them. They fought together, died together… sometimes for a cause they didn't know. Sam had been the best display any of the Autbots had ever seen. Facing Megatron alone on the top of the building was a true act of bravery. And he was just a teenager.
And he had saved them all.
Maybe there was goodness in them, like Optimus had said.
It just took Ironhide a while to realize it for himself.