This is a quick theory. We’ve confirmed that Miles was born on the island, and we see him looking at himself as a baby. I believe that he may have gone too close to himself, or he may get closer in the near future.
Remember what happened in the Orchid Out-take video at the 2007 Comic-Con? If you don’t you can watch the video here:
What that video establishes is that future versions or people/bunnies shouldn’t touch or get too close to past versions of people/bunnies. We don’t know what would happen. Perhaps the future version would die, perhaps the universe would explode. I think that the I believe that he got too close to himself and therefore screwed up his brain and let him talk to dead people. That’s that.
Not Confused Just Lost 
Yes, it was easy to surmise Miles’ story. What we get from the Miles’ backstory (or, is that frontstory — time travel makes my head hurt), is that his mother felt something for her husband, but Miles always felt abandoned.
Miles will eventually tell his father who he really is and what will happen (as far Miles knows –all Darhma people die). But because he is from the future, his dad accepts the String Theory of time (as it can loop and twist but it remains the same skein). Because Miles is alive and tells dad that mom was too, Dad makes the heartbreaking decision to “excommunicate” Miles and mom, thereby saving their lives and allowing for the continuity of the string. Mom won’t understand, but there you are.
How does Miles have this power? Maybe because he was young enough to be on the island, he incorporate some of the island’s powers and then has it activate as he gets older and away from the island.
*** Wild Ass Theory ***
Miles will talk to his dead dad sometime this year or next and find out what happened to him. I think he’ll be the one who kills Ben as revenge.
I hope that Miles gets to talk to his dead dad. I think that’s more likely than telling Pierre the truth when he’s alive.
And I hope that Miles doesn’t geet to kill Ben too soon, I want Ben to be around for most of the last season. Honestly I can’t imagine Miles killing anyone. Then again it didn’t seem possible for Charlie or Hurley to kill anyone either.
– izi
Interesting thought which brings me back to the bunny and the previous time travel drawbacks…
Daniel’s girlfriend is “lost” in time and is shown bedridden. Maybe that is because she became un-anchored and has no point of reference. Just like if you would be blindfolded and deafened, then taken to an unknown location. After which being restored, you’d be disoriented until you found your reference points. Unfortunately it seems that you have very little to anchor onto when you are displaced in time (and not neccesarily by location).
Desmond was able to find reference through Penny (great time/love story there) and was able to accomodate the time shifts. (Which makes him outside the linear reference because he can “remember” things that occur in past real-time.
Now, I am beginning to suspect Miles has this gift because his older self will be in such close proximity to his younger self that his memories will somehow transfer themselves to the younger. In effect, he will hear the dead because he has already experienced these events and will become his gift. Sorta like we view our own deja vu, we feel we have experienced something before as it happens. Like my first comment, he is still traveling on his own skein (timeline), but it becomes too close to a point near the beginning of his own line as the occupy nearly the same space and time.