Why did Paul have to die at the end of the novel?

There were a number of reasons for this. First and foremost, there had to be a reason for Adrienne to tell the story to her daughter, and it had to resonate with her daughter. By having her own tragic story in her past, Adrienne could sympathize with Amanda, and Amanda could realize that it's possible to move on, no matter how much grief a person feels.

Also, of course, there is the theme of love and sacrifice, which was inherent to every aspect of the novel. But in order to sacrifice, something must be lost. Both characters were willing to sacrifice the time they would have with each other for the sake of their children, (Paul heading off to Ecuador, Adrienne going back to Rocky Mount) and I think that sacrifice in inherent to good parenting. Both Paul and Adrienne loved their children, and the sacrifices they were willing to make made them nobler as characters.

Also, of course, I write tragedies, and I do that to create poignancy for the readers. Hence, from the very beginning of the story, I knew that Paul had died in Ecuador. Had he lived, he and Adrienne would have been together, but there would have been no way to create the emotional impact necessary to make a short novel memorable. Had Paul and Adrienne told the story to Amanda together, for instance, (for an entirely different reason of course - maybe Amanda was simply wondering whether she would ever fall in love) the story would have come across as self-serving, rather than as a lesson. In that instance, Amanda would already know that love is possible, and logically, that would be one of the reasons she was upset. Her mom found love, but she couldn't. So why would Paul and Adrienne have to tell the story? In the end, it might make Amanda feel worse.

In each "alternative" version, I kept coming back to that sort of problem. Logically, there was no reason to tell the story unless it resonated. In order for the story to resonate, there had to be a reason for it to be told, and a certain similarity so the listener (Amanda) could learn a lesson.

In addition, I write dramatic fiction. Dramatic fiction should represent the realities of life, and the best dramatic fiction allows the reader to fell a number of different emotions - happiness, empathy, anger, frustration, sympathy, passion, and of course sadness. All of those other emotions are triggered in the novel as well, it's just that most people seem to remember only the final emotion -- sadness. But always remember that there would have been no sadness had the reader not cared for the characters, and to do that, the reader had to understand and sympathize with them.

And finally, always remember that all great love stories -- in novels or in life -- must, by definition, end in tragedy. If there's no great love, there's no great loss. I read that somewhere (I can't claim to have originated that statement) and it's guided me throughout each of my novels.