The Alt is the Island's Prologue by RoyVecchio

In this theory, we believe that everything we are seeing during the "flash-sideways" is actually a prologue to the events that occurred on the island and in the flashbacks of the first few seasons. In other words, the flashes we are seeing now are what Jacob has seen before in the mirror at the lighthouse.

With only limited interaction in the flash-sideways so far this season, we can still begin to pick up on a trend of characters coming to peace with things in their lives. In her flash-sideways, Kate put the needs of Claire above her own and helped her through the issues with her baby. In Locke's episode, he has come to terms with his disability and found peace in his relationship with Helen, no longer feeling inadequate. Also in Locke's episode we see Hurley continue to have a streak of good luck, and attempt to help Locke out by giving him another job. This past episode with Jack shows us that he has had a son and rediscovered a new relationship with him while simultaneously coming to terms with his own relationship with his father.

These sideways flashes are much different than the original flashbacks from the first couple seasons. In the original flashbacks, these characters' personal lives are a mess and there is nothing left for them in the off-island world. There is nothing to suggest that Jacob would see anything in the characters worthy of a candidacy. However, if Jacob had seen these flash-sideways before the first events of the island, he would have ample reason to believe these people are worthy of filling a role in his plans. He then interfered at earlier times in these characters' lives, which is why we see the bad versions of their lives in the original flashbacks and not the harmonious lives from the flash-sideways.

To summarize, the "flash-sideways" are, for lack of a better word, the prologue to the crash of Oceanic 815 and the events of the island. Jacob has already seen "these flash-sideways" in his mirror in the lighthouse and approved these characters for candidacy. He then went back earlier into these characters' lives and intervened to change their course, so that they would all eventually end up on the island. This intervention is what spawned the lives we saw in the original flashbacks and the subsequent crash of Oceanic 815.