Okay so this is a two part theory. The first part came to me while I was driving and thinking about Lost and holy crap the lightbulb went off in my head. One of the major questions I my mind was why exactly did the Man in Black need to have a body in order to look like them? It seemed that he was able to scan people's minds and appear as other people to them as he did with Eko (appearing as Emeka & Daniel the boy) and as he did with Kate (assuming he was the horse). We've also seen him actually appear corporeal as Christian, Yemi, Alex & Locke. With that situation he needed a body to be able to walk around and touch things and be convincing. The question was always why? Then it hit me. His ability is almost identical to that of Miles. Miles needed a body present in order to be able to listen to the last thoughts of a person before they died. In fact, inside the foot, the Man in Black tells Ben in detail the last thoughts that John Locke had before he died. So the Man in Black needs the dead body to fully scan it and collect the memories before he can become them. If he scans a live person, it seems he can only become one of their memories, but a dead person is all memories so as long as he scans the body, he can assume their memories. This also leads me to believe that the Man in Black was the one behind the strange dreams on the island as dreams and memories are extremely similar in nature.
So this brings me to my next point about Specialness. Miles is special in a way similar to how the Man in Black was special.
In fact let's make the list of special qualities and compare:
Communicates with the dead:
Miles, Hurley, Man in Black (he saw Claudia as a kid)
Heals people:
Jack (he healed Sarah), Jacob
Summons things:
Walt (birds, polar bear), Jacob
Wills things to happen:
Hurley (willed the van to start), MIB (willed the wheel to work)
Able to tell the true nature of people:
Sawyer (knew MIB wasn't Locke), Achara (saw Jack's true nature), Jacob (somehow knew everything about everyone even off island)
In tune with the Island:
Locke, Jacob, MIB
Impervious to the light, time travel:
Desmond (seems his alone in his specialness)
See where I'm going with this? It seems that each of the special people either take after Jacob or the MIB. It's almost as if they were selected as candidates because of their specialness, because Jacob felt they would help due to the similarities between them, himself & MIB.
I at first thought that maybe the specialness was due to Jacob's touch, but thinking about it, Hurley talked to dead people before Jacob touched him. And Locke drew a picture of the MIB when he was just a boy.
That brings me to my next theory, which is very related.
Claiming.
The so-called sickness that Rousseau referred to was described by Dogen & Lennon as claiming. Personally I think that Jacob & MIB both did their own claiming and that was how they played the game. Jacob touched people and brought them to his side. MIB also touched people and brought them to his side. When Jacob touched people he gave them a gift (he made Jack guardian, he made Richard immortal). When MIB touched people he gave them the so-called sickness which also gave them a gift in a sense (he made Sayid emotionless, he made Claire essentially lose her mind). And it was made pretty clear that the MIB did in fact touch people. In fact, they addressed the issue when people refused to take his hand, as when Kate & Sun didn't. Sawyer took his hand, as did Desmond. Personally I think Desmond was impervious to both Jacob & MIB's touch since he's impervious to the island's energy. The touch did nothing. But the touch to Sawyer did bring him to the dark side. In fact he was with the MIB for a good portion of the beginning of season 6. It was during his episode Recon that Sawyer made the choice not to turn to the dark side (or as Dogen put it tip the scales the wrong way). He instead chose to play both sides of the fence. Because he chose not to embrace the MIB and the dark side, he let his scales tip to the light and was not claimed. It was all about choice. Jack had the choice to be guardian. He didn't have to. He chose to let the light in. Sayid had the choice to be claimed by the MIB. He let his scales tip to the dark side and the darkness went into him, took over his heart and he was claimed. When he was at the well with Desmond, he made another choice and I think in choosing not to shoot Desmond, he let his scales tip back to the light, so when he was on the sub and chose to sacrifice himself for the group, he was again back towards the light. I think this is why his sideways self was so conflicted with being good/bad. He had not fully resolved this at the moment of his death and needed the extra push in the sideways world before he could move on.
I think the character in which this good/bad struggle was so evident was Ben. Ben appears to have been claimed by both Jacob and Ben. He was brought to the temple by Richard and healed and like Richard said, was never the same. I think both laid claim to him and that is why he kept going back and forth between being good and evil. This was most evident in the sixth season. When he was with Ilana and he opened up to her and she accepted him, he was redeemed in the eyes of Jacob, he let his scales tip towards the light. But later in the season, he let his pride and hate get the better of him and he let the scales tip to the darkness and he shot Widmore and became a slave to the MIB up until the end. When he stepped in front of the falling tree to save the other Losties, I think Ben tipped back to the side of the light (we never did find out how he got out from under the tree) and when he accepted Hurley's job offer, his scale was fully at the side of light. Since his island life was so conflicted between good and evil, even more than Sayid's, I think this is why he decided that he couldn't move on with the others, and chose to stay in the sideways and work things out with Alex.
I'd appreciate any thoughts or comments.