Quote Originally Posted by February
Maybe because MS takes time to develop, or there must be some mental stage one must surpass to actually be able to use it. We still dont know what effects a person must go through, (other than the condition) to actually use it. I doubt its just kill the person and BAM I got it. Kishi didnt give us an alternative reason as to how MS can be acquired, and so far, killing your best friend is the only option as of now. And the closest person I can think of as best friend to Kakashi is Obito. So thats the closest thing we can guess to at the moment
Kishimoto gives us other venues to look at it differently.
Especially with Sasuke, Itachi tells him that he must kill his best friend to obtain Mangekyou Sharingan, but is Sasuke going to kill Naruto, I highly doubt it. Sasuke says I'll do things my own way and obtain things to kill him.

Mangekyou is probably one of the most needed techniques Sasuke needs to be able to fight Itachi more evenly.

So, I don't think gaining Mangekyou through killing your best friend is the only method, there is, but it's the only one we know of at this time, it gives us more possibilities of obtaining it, especially with Kakashi having it.