Also aside from my hoe thoughts on the Sharingan, I could also see the Sharingan being involuntarily activated due to fear, because biologically speaking the only reason the Sharingan would ever come into existence in the first place would be to help the Uchiha survive the many dangers of the Warring States period. My alternative theory as to why Itachi really went blind is because he probably lived almost every moment of his life after leaving Konoha in fear. Fear of being discovered as a spy. Fear of Danzo not keeping his promise to keep Sasuke safe. Fear of being attacked while he is sleeping or eating or using the bathroom. Fear of Konoha being destroyed while he is too far away and perhaps even unaware to do anything. Itachi probably never told Kisame that he actually couldn't just his turn his Sharingan off whenever he wanted to, not always anyways. However Kisame, being half shark, could probably still smell the fear on Itachi but respected him too much to tell him that he knew that he was scared. Kisame telling Itachi that he uses his Sharingan too much during their infiltration of Konoha really was his gentle way of trying to reassure Itachi that he would be ok, he didn't need his Sharingan on because Kisame could and would protect him. But of course Kisame didn't know that Itachi couldn't turn it off when he was scared.
Even in regular humans our pupils will get really big as an involuntary fear response, and what is the Sharingan if not just some weird looking pupils? The same biology should apply to them.
