Since Didyme's sudden death, Marcus hadn't had much to live for. All that had kept him going through the countless years was the undying devotion which he held to the Volturi, but even that was starting to wane. He was, well, he was yearning for something more.
Just because the Volturi was the closet thing to royalty in the vampire world, just because they were the unofficial kings, didn't spell out happiness. Going to Forks to deal with the 'immortal child' was just another act to keep their world secret.
No one could have known that everything would change, and give Marcus back his purpose.