Okay, I'm a newcomer to the show but I have seen all the episodes and I was wondering what Jack's life was like before he met Tru and this is my theory.
He was a normal guy with a normal life before he almost died (heart stopped for three and a half minutes) and for some reason he remained alive. A few days later, he relived a day when someone asked Tru for help, for some reason, Richard, while being alive was no longer the person that was shot back in time aswell, possibly because he was responsible for the murder of the previous person reponsible for the day reliving (without someone to be asked, the other obviously can't go back either and maybe that means it's permanent for both so others are chosen every ten years).
Jack got really worried and kept leaving his job because he thought it was stress but finally after a few months he checked himself into an asylum where this kept happening since people were still asking for Tru's help. When Richard hears someone's reliving days, he goes to the asylum and explains everything to Jack, who after his near death experiences realises this is why he was brought back. Richard assumes Tru got her mother's gift and sends Jack to make sure before he gets going.
On the second day of his interview, Davis mentions his previous employers and Jack realises Tru must have warned him so he accepts the job to keep an eye on Tru. After a few day relivings with no bodies coming when they did previously, Jack stepped up to the plate and made sure he preserved fate, interfering to make sure that the girl Harrison saved in Two Pair kills herself. Once Tru discovers he was responsible for these and other deaths, he becomes more involved since his cover is blown and can more easily preserve fate but can also more easily be beaten, as seen in the first season two episode.
However, one day he is asked to help someone and the rules are seemingly reversed until he realises that it had to look like an accident and even though Tru got the money the girl needed and she no longer had to kill herself or make it look like an accident, Jack does his job and drops her so he does what was asked of him and did his job but still realised that he could no longer get close to any of the people who were intended to die.
That's my theory. What do you guys reckon?