Board Thread:Naruto Discussions/@comment-4682609-20140806081616/@comment-24071637-20140810192606

Steveo920 wrote: Technically, his left eye never had the Sharingan to begin with. It was always Obito's. But considering that Kakashi grew up and developed with the said eye, even awakening the Mangekyo form, it does stand to reason that he might have integrated some of his essence. Danzo, who also had a "borrowed" Sharingan, couldn't maintain a single eye for very long with each usage, ultimately needing to get a new one periodically. Kakashi's however never showed any signs of permanent damage.

All that is true, Kakashi lost his left eye in the same incident where he got the scar. And it stands to reason that the more you own a sharingan it gets more used to you, because Kakashi has had his Sharingan long before Danzo got his from the Uchiha Massacre.