Maybe a particular emotion has to be triggered and has a set amount of chakra embedded in it? Weird as it sounds, I'm starting to get an idea...
Both Kakashi and Itachi have it, right? Kakashi must've gained it through a remorseful state of mind. Every time he uses sharingan he must be, subconsciously, thinking about Obito. Itachi obtain it through killing his friend. As he was killing him he might've experiences rage and detestment toward his friend and his entire clan.
That also might be a theory to explain the different MS?
*Puts up flame shield*