I think the Sharingan open purely base on emotion. the determination to protect friend at Obito. The determination to protect Naruto at Sasuke, the determination to battle with Naruto (3 dots). The determination to kill as Itachi (Mag. Sharingan). Maybe there is a way to open Mag. Sharingan without kill closest friend after all, as long as it have the same determination with killing your closest friend. don't you think?