I have often imagined what it would have been like if Jake and Bella had ended up together. Having looked at their personalities, what they have in common, their strengths and weaknesses, I have come to this conclusion:
A relationship between them wouldn't be anywhere near as easy as Jacob thinks it would be.
Remember, it is Jake who says that. Jake gets things wrong. Jake wants Bella. Of course he's going to think that. But I honestly don't think the information we have on their characters leads us to that conclusion.
Let's say Edward doesn't come back at the end of New Moon, or at least, by the time he does, Bella has settled for Jake, she seems reasonably happy, so he goes away without her ever knowing he was there. With that in mind, let's look at Bella and Jake and see what they like to do together:
1) Bella likes to sit in the garage and watch Jake while he works.
Fair enough. She likes to spend time with him. He's a nice guy, and they like to talk. But honestly, how long is her enjoyment of sitting in the garage watching him work going to last? How many relationships start out that way, with the girl just enjoying hanging out while her boyfriend works? How many times does it wear thin eventually?
Down the track, I see Bella taking a book in to read while Jake works. I don't think Jake's particularly into books, so he probably wouldn't be too interested in what she was reading. But maybe he would talk to her about it a little. I think he'd make a few jokes about it. He does that. Would she like that? Would she want him to show real interest in what she's reading? I certainly think he would try, seeing as it is important to her, but I think it would be hard for him to be interested in something like that long term.
And then if they had kids, she's probably going to be inside the house more looking after them. As time passes, Jake would keep spending his time in the garage, and I see Bella spending more time in the house doing whatever needed to be done there. After all, she can't really help him in the garage. She's too clumsy. She'd be likely to break whatever he was working on more than help out with it.
2) Bella and Jake ride motorcycles together.
This is bad before it even starts. Because what is the reason Bella wants to do this? To see Edward. This is classic rebound relationship stuff. It brings an unhealthy tone to their relationship from the start. You don't do things with your friend/future lover in order to see your ex and think it's going to lead to a healthy relationship.
But it's clear that Bella enjoys it. They like doing it together. But we strike similar problems. Bella is clumsy. Is she really going to be able to keep it up long term and not do herself some major injury? Charlie objects to it. Can she keep it up in the face of his objection? Even if she does, again, once she is pregnant, she will have to give it up. It's too great a risk to the baby. And after the kids are around, will she keep it up then? What if they want to ride the bikes? What if one of them is as clumsy as she is? Will she let them ride it if their lives are at risk? Not from what we know of her character. So will she keep it up under these circumstances? Not if it stops her kids from wanting to ride the bike. So no more motorcycle riding.
3) Bella and Jake go hiking together.
Although this is a good family activity, it has the same problem as point 2. Why does she take it up? Because she wants to see Edward. Again, not a sign of health in the relationship. And although it is something I could see her doing with Jake and their kids long term, again, she is very clumsy. This is one of the reasons she doesn't do it a lot anyway. It's always going to be a bit of a struggle for her when she's tripping and trying to keep up with everyone else. I'm sure she'd still do it for her family, and she'd probably enjoy it, but I can see self-sacrifice in it at least some of the time.
And from the information in the books, these are the only three things Jake and Bella have in common. It's not a lot, and all of them she will struggle to keep up because of her clumsiness. What's more, if they have sons (or even daughters) who grow up to be werewolves, how will they cope reading their father's thoughts, and knowing through them that their own mother was in love with a vampire, and probably still is? This would cause massive problems for them. Also, I think there would always be doubt in Jacob's mind that perhaps Bella was still in love with Edward. It would be somewhere behind her eyes. Would she leave him if Edward came back? That would put a strain on their relationship down the track, I think.
Okay, so maybe Edward doesn't exist. Maybe Bella never meets him, and doesn't get killed by Tyler Crowley's van, or by those men in Port Angeles. Maybe she and Jake just hang out, and fall in love.
Well, my argument for point 1 still stands. She's too clumsy, and not interested enough in cars for her to want to do that for too long of her own free will. In fact, the interest in cars is about the only thing she and Edward don't have in common. And as for point 2, with the motorcycle, would she ever have been interested in doing that without the desire to take risks to see Edward? I don't think so. She's too clumsy. So if Edward didn't exist, then she would never have taken it up in the first place. They could certainly hike together, but as I said, she's clumsy. It's always going to be that little bit difficult.
Bella is a bookish, pseudo-intellectual, anal-retentive type; a deep thinker. Jacob is a creative, build things with his hands, active, outdoorsy type of guy. Most of the things he likes to do she will find it difficult to do long term. There is no evidence that he has any real interest in the things she likes to do. Now, I'm sure they could still find some common ground, some compromise that could make it all work, but I think that the two of them holding a relationship together long term is not going to be as easy as breathing.