Sharingan Lore

26. Activation Control

I also think that while the initial awakening of the Sharingan responds only to negative emotions, the Sharingan itself may not necessarily only respond to negative emotions, but possibly to any passionate or intense emotion, both the positive and the negative.

Like Sarada's Sharingan involuntarily awakens when she wants to protect her mom, and it continues to stay activated for the duration of that scene even when the danger goes away, which makes me think that her Sharingan remains activated out of a sense of relief that she's safe and her happiness that her parents do care about her, as she cries tears of joy. Since those emotions of happiness and relief are so intense, it's a positive feeling, but one strong enough to activate the Sharingan, or at least keep it activated even after the initial danger Shin posed has passed away.

And personally, I think there is a reason for the anime frames focusing closely on Kakashi's Sharingan specifically after his accidental kiss with Hanare, the person closest to being Kakashi's love interest. After his kiss with Hanare, Kakashi's Sharingan shimmers with emotion.

For similar reasons, I think the focus on Sasuke's shimmering Sharingan eye after he catches Sakura to stop her fall seems significant as well, and one could speculate that romantic passion could be something that triggers an involuntary activation of the Sharingan. Though yes, that's just headcanon conjecture, since we don't really have enough information to back that, other than the common knowledge that it indicates a significant reaction any time an anime focuses on a character's shimmering eyes. 

I'm merely operating off the assumption that the Sharingan, as the eyes that reflect the heart, might sometimes have an involuntary reaction to any intense or passionate or strongly felt emotion, whether it is love, hate, fear, relief, joy, sadness, grief, etc. I think the involuntary reaction is going to be pretty rare, but involuntary activations do happen, as Izumi notes that sometimes when she gets angry, her Sharingan involuntarily activates, which is probably a common occurrence for Uchiha members, particularly younger ones who don't have a firm control of their emotions or for those who are extra emotional, extra-sensitive, or lack emotional self-control.

As the eyes that reflect the heart, it seems reasonable that the Sharingan involuntarily responds to strong emotion of any kind, since emotions are wild, unpredictable things, difficult to control and manage. The Sharingan is a physical, biological manifestation of its user's emotions, much like tears are a physical manifestation of a person's sadness or quivering is a person's physical response to fear.