A king in exile
One of the unique things to consider when thinking about King Varian is the amount of experience he's had in his relatively young life. While before his father's death, he was as sheltered as any young prince could have been, witnessing Garona's murder of his father (and seeing her cut out his father's heart) and then having to be dragged away by Lothar from the burning wreckage of everything he'd ever known was just the first of many hard lessons Varian would learn in war and kingship. Varian has grown to manhood and rulership in a world where hostile aliens invaded and destroyed everything he ever loved. These same invaders even killed his own surrogate father in Lothar, as the great warrior drove the orcs back from Stormwind and humanity.
Yet when the leaders of the Alliance of Lordaeron were voting on what to do with the conquered orcs, Varian managed to find it in himself to vote against simply killing them all. Kings who had lost much less than he had, like Thoras Trollbane and Genn Greymane, urged that the orcs be purged, put to the sword so they could never recover and attack humans again. But Varian stood with Terenas (his benefactor who had sheltered him when Stormwind fell) and the Alliance even as others abandoned it. So Varian's personal loyalty to Terenas outweighed his own potential desire for revenge. Despite seeing an orc butcher his father right in front of him, despite knowing that orcs had killed Lothar, despite his memories of orcs burning down Stormwind, he still managed to restrain himself and vote to support Terenas.
Likewise, the complicated friendship between Arthas Menethil and Varian creates an interesting window into each, a way of looking at their worldviews. Arthas saw Varian as a potential friend before they met (there not being a lot of social equals to the son of a king), but upon meeting Varian for the first time, Arthas was intimidated by the look of naked loss the child-king of Stormwind bore. As they effectively grew up together, they did become friends, but to Arthas, their friendship always bore elements of rivalry. Arthas envied Varian's ability in combat, having spent his childhood protected. Varian understood his frustration. "My father tried to protect me, too. Didn't work." While Arthas saw Varian as a rival to contest with or an example to emulate, Varian saw in Arthas someone who would, if lucky, live the life forever denied him.
Varian lost his father to the enemy, and so when Arthas became a patricide, his crime was even more unthinkable to Varian because he'd thrown away the one thing Varian had tried to regain his entire life. In the process, Arthas killed yet another surrogate father of Varian's. King Terenas had caused the Alliance to help rebuild Stormwind and had seen Varian installed and crowned its monarch, with a father's pride in a son who had done well. In a very real way, Lordaeron was the home of Varian's youth and Terenas the steward of his childhood, the father who had managed to survive the horrors of war and give Varian an example of how to be a ruler.
The modern King Varian is not only shaped by these losses, he's defined by them. He is the king who does not truly wish to be king, who would most likely be far happier as a gladiator or a war leader in the style of the ancient Arathor leaders. Ironically, while his life experiences have shaped him to distrust and even revile the Horde, his temperament would be far more suited to being a Horde warchief than an Alliance king.
Loss follows loss
It's no wonder that the death of his wife Tiffin was so devastating to Varian. After returning to Stormwind and his coronation as its king, he spent very little time actually ruling. His marriage to Tiffin Ellerian was arranged and was not a love match. They initially didn't care much for one another, although we have as yet seen little of their courtship or how they came to love one another. Clearly they did, as we've seen them beaming at their son and Varian's very deep grief at her death.
The irony is in how she died and why. Varian's attempts to personally patrol his kingdom, striking down bandits and hunting for any sign of his father's killer, Garona, led him to leave power over the day-to-day running of the city in the hands of the House of Nobles. These nobles were less than eager to pay the stonemasons who had worked to reconstruct Stormwind. There are arguments back and forth on who is ultimately responsible for what happened ... Some claim the Stonemasons demanded a staggeringly unfair price, others that the Nobles were parsimonious, and still others point to the Lady Katriana Prestor's hand in all this.
But Queen Tiffin
was a staunch supporter of the stonemasons. Therefore, her death during the riot that they unleashed on the city after the House of Nobles arranged for a small fraction of what they expected not only nearly destroyed the king, but it removed one of the strongest advocates they had and led to the formation of the Defias. A king in mourning was hardly able to out-politic the nobility. Nor was he truly in any mood to do so.