I think it’s important for couples to keep things fresh regardless of whether they are fictional or not. Boredom and complacency are two of the biggest relationship killers. A couple still needs to be able to surprise each other even after years or, as is the case in paranormal fiction, millenia together. It doesn’t have to be anything crazy but an unplanned weekend getaway or even learning a new hobby can do wonders for a couple’s mental health. Also, it’s crucial that they make time for each other which is no easy task when you’re busy slaying vamps and unraveling some evil villain’s master plan but even supes need a vacation from time to time (even though the bad guy usually ends up crashing it anyway).
Conflict is also important to the success of any relationship just so long as it’s not overdone. In the early stages it’s normal for a couple to fight over what I like to call the “big stuff” because they are still learning about their partner and aren’t yet sure whether their union will last so they test each other. This of course often leads to make-up sex which is sizzle enough in itself! However, once a series hits 5+ books, I expect the lovebirds to have found their comfort zone and to only fight over little things like who left the cap off the toothpaste. Ok… so most paranormal couples don’t argue over anything so mundane as dried out fluoride but you get my meaning. If they are still arguing over the “big stuff”, it gets old real fast. I absolutely HATE it when a duo continues to have the same fight over and over again; without conflict the relationship becomes boring to read about but with too much it can be overkill too. There’s a fine line.
In my opinion, the key to making a long term couple sizzle is growth. The characters, story and world evolve from book-to-book so it’s important that the protagonists’ relationship continues to change as well.