Originally Posted by The Special One
That was not explained in the manga however. According to data base there, Kishi states that we should look out for how Kakashi obtained the Mangekyou Sharingan, as it would be revealed in the following chapters.
MS comes from killing or watching someone close to u die. Sasuke saw Itachi die but dint get MS until he cried. Killing someone close to u or watching them die would likely bring tears to one's eyes. I think that Kakshi finally shed a tear for obito or he either shed a tear or 2 when he saw Rin die, and learned to use MS later since it wasnt that simple for him to immediately use since he isnt Uchiha.
I think that Kakashi's statement about MS killing him was more toward meaning that using it too much completely drained him of chakra, which is likely one's life force as well. Sasuke was said to be out of chakra and about to die after his fight with Itachi. If u use up all the chakra that u have, u are likely left on the brink of death. Kakashi wont die but he is on death's doorstep