Thinking about some of the more prominent themes of Lost, I have to assume that Jacob and MiB were placed on the Island as a form of punishment, and a chance for redemption. This happened before any of the subjects were introduced to the Island - somehow their punishment traps them on the Island (at least in the long run), and forces them to test the human condition. Once a point is proven, once someone realizes the mistakes of his past, then it ends ('it only ends once, everything up until then is just progress').
I am thinking that the trials and hardships endured by the 815 survivors, the DI, and probably everyone brought to the Island runs parallel to the lessons trying to be taught to the higher powers on the Island. In other words, Jacob and MiB are being taught the same lessons that the survivors are learning. The survivors are put into impossible situations to teach them, to see how they react, which in turn teaches Jacob/MiB. Or perhaps teaches just Jacob, since MiB seems to be the antagonist.
I know I made quite a few assumptions here, but humor me!