Anyone think that Jack will find out who Sarah's (his ex-wife) is seeing? I have a feeling he has something to do with getting Jack on the island. It was the only way to ensure Sarah's safety. If she had told Jack who it was then I think Jack would have taken vengeance on him.
That's a pretty good theory. Remember back to Season 3: Episode 1 when Jack gets bailed out of prison, I think Jack may have briefly seen the guy who Sarah is now with.