T'Pring was a Vulcan female who was bonded to Spock as a child, and was to marry him and mate with him, as he was suffering from pon farr. During the wedding ceremony, she executed her right to claim kal-if-fee, a fight between two males for the right to mate. She chose Kirk as her champion, and he accepted, unaware that it would be a fight to the death. After Spock seemingly killed Kirk (though it was actually a trick by McCoy to save the captain), he was brought back to his senses, and T'Pring revealed what her plan was.
She had no feelings for Spock, and didn't wish to be married to him, and was instead in love with another Vulcan named Stonn. She was counting on one of the two dying, but it didn't matter which one; she reasoned that if Spock won, he would reject her for having chosen another, and she would have Stonn. If Kirk won and killed Spock, he would not want her, and she would still have Stonn. However, should Spock still accept her, he would likely be gone for several extended periods of time following his career in Starfleet, thus allowing her to be with Stonn.
In essence, her plan involved the death of one man at the hands of his best friend, and she would've cheated on Spock if he chose to be with her. Spock 'congratulated' the logic of her plan, but cautioned her that "having is not so pleasing a thing after all as wanting".