On 28 June 1914, in the city of Sarajevo, a Serbian nationalist named Gavrilo Princip found himself in a position to assassinate the heir to the Austro-Hungarian throne. He took the chance, firing two shots at Archduke Ferdinand.
The first bullet missed the Archduke and struck Ferdinand's wife, Sophie. It was a gut shot that did severe damage to her. The heat of the first bullet speeding down the barrel of Pricncip's gun caused the slightest warping of the metal. His second shot became stuck at the end of the barrel of the weapon, sending it flying form his hand. The assassin was quickly subdued after this.
Realizing the severity of his wife's wounds Ferdinand ordered the car taken immediately to hospital. They were too late, however, Sophie died shortly after getting inside.
The worldwide outcry of the assassination hurt Serbia's standing in international affairs. The stronger calls for vengeance in the Austrian court were countered by Archduke Ferdinand himself. He claimed full right to determine what would be done since it was his wife that had been murdered. He had several heated arguments with Emperor Franz Joseph over the situation. The Emperor finally gave in to Ferdinand. Serbia was to face no violent retribution, even when their connections to the assassin was confirmed. Instead Serbia was diplomatically isolated and even lost some of its traditional support from the Russian Empire.
On 17 December 1916 Franz Joseph died of natural causes and Ferdinand ascended to the throne. With his wife dead and his son unable to inherit the throne the new Emperor was able to devote all of his energies and political capital in reforms. He worked tirelessly to strengthen other ethnic groups in the Empire to cut down on nationalistic feelings that could threaten the Empire. He also used these newly empowered groups to counter growing Hungarian influence in the court. A stronger Slavic element in the Imperial government was designed to counter both Hungary and Slavic nationalist groups outside the borders of the Empire. With improving conditions for Slavs in the Empire nations like Serbia had an increasingly difficult time causing trouble.
In 1924 he would proclaim the Triple Monarchy. The Triple Monarchy would not last long. Hungarian rebellion, an 11th Russo-Turkish war, and numerous other conflicts growing in the Balkans threatened the stability of the region. The 1920s & 1930s would be an age of chaos for south-east Europe as well as the collapsing Ottoman Empire.
One of his other achievements as Emperor was the commissioning of a new dreadnaught for the Imperial Navy, the Duchess of Hohenberg, in honor of his wife.
Of course in the original timeline both Archduke Ferdinand and his wife were assassinated. That spark eventually led to the First World War.
In this timeline the war drums beating a little less and a strong-willed man worked to keep his nation out of war. Ferdinand was strong willed. He married a lesser noble for love in defiance of the Emperor and at the cost of having his children ever reaching high titles. He is a man who would fight for what he wants.
Of course he could just of easily have decided to wipe Serbia off the face of the map after having his wife die in his arms that day. We may have ended up with nearly the same conflict we had in the OTL. It would depend on how he dealt with his conflicting duties to his nation and to avenging his wife's death.
The photo of Sophie above is another public domain image borrowed from Wikipedia.