They were living in a fantasy...
They kept thinking everything was going to be great once they got back in the same city but it never was. The movie showed how everything was great with Sam and Simon. Sure they felt like something was missing when they were with these people but as soon as Jacob and Anna were back together it was weird. I never saw two people who were in love be so distant with eachother. They just weren't meant to be. They were holding on to some kind of fantasy in their minds. In theory it sounded like they needed to be together but in practice it just didn't feel right. They didn't fit in eachother's lives anymore, sad but true. I think this happens in real life a lot. People just can't let go of eachother when they know they don't work. It was sad seeing Jacob go back to Sam and then her leaving again. I felt like he really dragged her through the mud. And poor Simon. I felt terrible for him when he proposed. Ultimately, the relationships they were in seemed perfectly fine. They looked happy but they were alway hung up on what could have been rather than what was.
The movie was left on a weird note since they were married. I have to believe that they ended their marriage sometime after.