Gripped by the Vulcan mating drive, Spock went home to marry T'Pring, who chose a ritual combat in which Spock appeared to kill Kirk.
In 2267 Spock went through the pon farr, a Vulcan biological drive that could kill him unless he took part in a mating ritual. Kirk disobeyed orders to take the Enterprise to Vulcan for the ceremony, overseen by the respected T'Pau. Spock's bride, T'Pring, claimed the right of challenge and chose Kirk as her champion, knowing she would get the man she wanted, Stonn, whatever happened.
The fight became a fight to the death. McCoy gave Kirk an injection that made him look dead, which ended the fight and Spock's fever. When Spock returned to the ship and found Kirk alive, he showed unguarded joy.