All of this has happened before (and all of it will happen again) by HumeandHoltz

Some of this may have been posted before and if so, I apologize. It's worth noting that this is the first theory I've posted, so if anything I say is unclear please feel free to comment and I'll try to answer as best I can.

I've been trying to use clues and information we've already gathered from previous episodes in order to determine what the flash sideways could be (and what they mean for the end game of the show). Something I keep coming back to is the episode after the hatch has exploded when Desmond has seemingly gone back in time and has a chance to do things differently. We know (or can assume) that the electromagnetic energy sent him back to the time in his life when he made a choice (not to propose to Penny) that sent him on a journey to the island. Some important parallels with the flash sideways are that he initially doesn't remember the island, but has a sense of deja vu and his meeting with Charlie is what triggers his memory of the island to come back.

In the ring shop scene, when Desmond has a chance to purchase the engagement ring and change his life course, Eloise Hawking steps in to course correct. I guess my question is, what if she hadn't? Would Desmond have proposed to Penny? Would his existence on the island have become negligible? Would Radzinsky have lived long enough to push the button? Certainly the 815'ers never would have crashed. It all comes down to a choice made by Desmond and the fact that Eloise knew she needed to push him in the right direction to make it.

The next puzzle piece is Dan Farady and his mother (Eloise Hawking). I think we've been given two substantial clues from the two of them. The first is Dan's flash back where his mother raises him to go to the island, because in her past she has shot the adult Daniel. So now she's raising young Daniel to not let that happen AGAIN. If she knows to raise him because of what's happened - then it's already happened at least once. The idea of a time loop makes sense in that Eloise has knowledge of the future (what choices must be made) and because of the scene where Daniel sees the footage of 815 at the bottom of the ocean and cries for reasons he doesn't understand.

So if one choice leads to a time loop, then there must be another choice that leads to the end (It only ends once, all the other times are progress). Which makes me think that if Eloise knew to direct Desmond on a path that leads to the island, that there could be a divergent path where he instead makes the choice to marry Penny. And in that reality, 815 never crashes. It's possible that the side ways reality is, instead of a world without Jacob (as some theorize), a world where Desmond chooses Penny the first time around. It gives the ability to explain why the island was there and why it sunk (Radzinsky never gets a replacement). It explains why elements from the Losties pasts remain the same. And it nicely ties in why Desmond traveled back in time to that specific moment in his life and why Eloise was there to intervene.

Thanks! Obviously this isn't air tight, but I like the thought that Desmond is the crux of the entire show.