let´s think about a little different: May be the life they had before, which we see in the flashback aren´t real! They all survivors of a war, nuclear bombing, end of the world or because they human´s come down of a new virus. So and the people how escape have to live with old computers because they make sure they run, on an old island. They send help signals for more then 16 years but no one can help them, because the world is in chaos. They have wishes and dreams and maybe they have a possibility to use old or new human practise to life a alternate life with no trouble in life or find eat. One wants to be Rock star, one want to be doctor (or they need one so he have to practise before), the other want a to her father, one want to be a father and so one. The things that happened are a mix of their wish life and the real one, but they don´t remember what is the real one. Kate and Sawyer are always together but they don´t know it they feel it. But Michael is searching for his son, but did he really have one? Jack has to operate on a human on the same way he practiced before.
It takes too long for theories but think about this......