Why couldn't he just stay with Sadie in the past...?

I agree. It's the problem with this show, how would anyone know for sure. The nutcase/Yellow Card Man implies that you can't change things(he's stuck in a loop where he is trying to save his little girl but she dies every time), so the viewers are supposed to think that Sadie would die every time as well. But it's impossible to know whether she would die if Jake stayed or not.

***Possible spoilers****

I haven't got far in the show yet but the book basically says that everytime someone goes back it sends ripples through space and time and creates a parallel universe. So the more you create that deviate heavily from the actual past the worse it gets and you could destroy the future entirely.

In the books the Yellow card man is actually part of a group that patrol these portals (yes there is more than one). And each time someone comes through it in a why hurts them in the book by about the second time Jake comes through they yellow card is black and he's dead. When Jake stops Oswald and finds out its much worse he goes back through and there is a new yellow card man that tells him all of this and basically pleads with him to not change anything and let the diner get demolished so that this particular portal can close for ever.

The point was that you can't change the past. Him just being with Sadie would have ended up bring her harm and/or the rest of the world in some way, no matter how good your intentions are.

Kind of a grim message to end the show on, but true in a way. We have to accept our past and try to move forward.

Simply put, she would likely die, every time.

As you saw in the end, she is supposed to have the life we got to see right there, and him being with her would upset that. Not much, not anything that would destroy the world, but the "Time Entity" or whatever I should call it always finds a way to reset.

Just think about living your happy life with an oblivious Sadie, wondering where, when and how Time is going to set things right... this time around. It would be a nightmare.

I agree it doesn't make sense. First of all, "the past" didn't want him to save the president either but he managed to do it. So, even if "the past" didn't want him to end up with Sadie, that didn't mean that he couldn't do it.

Just imagine this: he spends some time with her, they fall in love and he takes her with him to 2016. According to the disturbed man, he wouldn't be able to do it because she would die anyway. But he loved her so he would always try to do it, at least once. If she died again, he could just reset it and give up. Why wouldn't he try it at least once? That doesn't make sense. Even if she died in 2016, he could just go back to 1960 and com back to 2016 not taking her with him this time.

Except, every time he comes back to 1960 and lives through to 1963, he ages another three years (five years in the book, because it was 1958). After, say, three times of trying to make things work out, he's now almost ten years older, and might no be as attractive to Sadie...

I didn't get that either. He could've been with Sadie if he wanted to. What bugs me about the ripple in time theory is that he even went back at the end and lived until Sadie was old just so he could witness her little speech at the school and have a last dance with her. Where did he live all those years? How did that alone not affect or alter time? He wasted years in the past just waiting around for that moment to see if she would end up happy, even knowing it might alter time. Why not live with her that long and then see how it affected time?

Essentially the rules were something like 1 minute in future time is 1 year in past time so he could've tried multiple ways of doing things every night. And even if being with her did screw up time drastically, he could just undo it every time. He would essentially get to be with her multiple lifetimes over and over. It's like living forever. Plus with some well placed stocks he could make decent money and not even need to work, or work as a teacher as he did before since he likes it. This just didn't make sense to me.

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 Idiots

Everything is interconnected. The slightest change, here or there, can have cascading effects (the old butterfly flap in time travel lore) and he had to settle in the time line that most mirrored the outcome he desired.

The Dude abides.