Why are these books so addictive?
First of all, I should note that I am not, and never have been, a reader of romance novels. I am one of those readers who "wouldn't be caught dead in the Romance section of a bookstore". <g> Nothing wrong with romance as a genre, of course -- and no offense meant to Jo and Eve and other romance writers here on the forum -- but it's just not the type of story that appeals to me all that much as a reader. If OUTLANDER had had a romance-novel-type cover, featuring a barechested Fabio-type and a swooning female, I can assure you I would never have picked it up in a million years.
I read OUTLANDER the first time, in 2006, because I'm a longtime fan of both time-travel stories and Big Fat Historical Novels. The idea of a time-travel story set in 18th century Scotland (a place and a historical period that I knew nothing about) intrigued me. I had no idea, on the first reading, that the relationship between Claire and Jamie would become the main focus of that book. In fact, on my first reading of OUTLANDER, I paid relatively little attention to Jamie in the first part of the book <wry g>, and I was reading so fast the first time (eager to find out what happened next, to Claire) that it didn't occur to me that Jamie and Claire were going to be together until just before the wedding. That sounds ridiculous in retrospect, but it's true.
OK, so what was it about OUTLANDER (and the other books in the series) that got me hooked?
1) The characters.
The people in these books are very human, and the way they react (in any number of situations) is both realistic and true to their personalities. They have vices and faults and flaws, and I think those imperfections are what make them seem like real people. Yes, Jamie is larger-than-life, but he also has his share of flaws. He makes mistakes, sometimes with devastating consequences. (Beating up Ronnie MacNab, for example, which led directly to Ronnie betraying him to the Watch, which led eventually to Wentworth and everything that happened there.) And Claire's unfamiliarity with the culture and customs of the 18th century gets her into trouble on multiple occasions in OUTLANDER, as well as in some of the later books.
And because these characters are portrayed so realistically, many of us have come to think of them as real people, and to react to what happens to them as though it were happening to a close friend or family member. You laugh and cry with these characters, you're afraid for them when they're in danger, and so on. I'm not a person who normally cries over fictional characters, but on my first reading of the series, when I found myself crying through the last part of OUTLANDER, and again at the end of DRAGONFLY IN AMBER, I was just stunned by the emotional power of Diana's writing, that it can make me react like that. (To this day, I don't know how she does it. I've heard her explain this, more than once, but I still marvel at the way she can cause such strong emotional reactions among her readers.) This is another reason why I became so thoroughly addicted.
2) The conflicts.
I love watching the sparks fly when Jamie and Claire argue -- for example, during the infamous wife-beating scene. Two very stubborn people, each absolutely convinced of the moral rightness of their own positions -- and both of them are right, in their own way. They have to learn to make compromises, as any married couple does.
Some of the decisions Claire faces in OUTLANDER are just heart-wrenching. Return home to Frank and leave Jamie forever? Leave Jamie in Wentworth to die at the hands of Black Jack Randall, or find some way -- against impossible odds -- to get him out? Maybe it's different, reading these parts of the book from a guy's perspective, I don't know. <g> But I always have to ask myself, what would I do in those circumstances?
3) Diana's wonderful writing.
Diana's use of language in these books continually amazes me, even after multiple re-reads. <g> The lyrical descriptions, the humor (I don't think I would enjoy these books half as much without the occasional flashes of humor, both in dialogue and in the characters' thoughts), the use of obscure or unusual words, just for the fun of it <g> -- all of these things add immensely to my enjoyment of the series. And the vast amount of detail in each book means that these are books that stand up incredibly well to re-reading. You can't possibly pick up all the tiny details the first time through, no matter how slowly or how carefully you read.
4) Jamie and Claire's relationship.
I can't deny that Jamie and Claire's love for one another is the aspect of these books that appeals to me the most. The idea of a love so powerful that it can outlast death itself is a very compelling one, at least to me. No matter what happens, no matter what adventures they go through or what traumatic events they endure, Jamie and Claire's love for each other never wavers, and I don't think it ever will. (That sounds mushy and romantic, and I don't intend it that way. <g> It's just that there's no way to explain the appeal of their relationship to me without using words like that.) Both of them have risked their lives to save the other. Jamie was willing to sacrifice his life (literally) to save Claire's. And Claire pulled Jamie back from the brink of death (and out of a severe depression) at the Abbey, using a combination of prayer, desperation, stubbornness, and sheer force of will. Such absolute devotion to one another is something I've rarely seen in fiction, and to me, it's absolutely irresistible.
5) The sex. Which is fun, no question about it <vbg>, and I enjoy those scenes as much as anyone else here, but IMHO it's a secondary aspect of Jamie and Claire's relationship.