I have been thinking about it, and it seems like most of the people on the island from 815 have done something wrong/illegal, or they have "issues". A few examples are: sawyer-wanted to kill anthony cooper, sun-tried to leave jin, hurley-possibly insane, locke and jack-"daddy issues", etc. etc.
Now, think about this: what if the island "selects" the people who go to it. What I mean by that is, what if the island selected that group of people for the reasons stated in the first paragraph, because it knows that they will be tested (e.g. by the monster, etc.). Just a little food for thought, but still . . .
(p.s. I'm still in the process of finishing (what's been made of) the series, so please excuse any incorrect information.