TRISTAN: Well, it’s just, with this being our last kiss and all, it makes me think about our first kiss. You know, at the party.
RORY: What? …
TRISTAN: You remember the kiss. In Act 1 at the Capulet’s masked party?
Tristan promised not to say anything to Dean about the kiss he and Rory shared at Madeline’s party. However, he finds a fiendish way to allude to it, by talking about the first kiss their characters shared at the Capulet’s masked ball in Act 1, and suggesting that perhaps Juliet should cry during their kiss. This serves to delay the actual kiss, as he leaves Rory dangling while he pops up to ask for opinions. She’s clearly dreading being kissed in front of Dean, and he doesn’t let her get it over with quickly.
Rory naturally panics at where this slow reveal may be going, and begs Dean to leave, as he is making rehearsal more difficult. This situation is basically all Dean’s fault. He shouldn’t be at rehearsal – Rory is doing schoolwork, and he is at best a massive distraction. Dean knows perfectly well that he and Tristan get on each other’s nerves, they almost had a fist fight the first time they met. The rehearsal could never be anything but a disaster with Dean there.
Once Dean is gone, Tristan ends up storming out of the rehearsal in a temper, ready for trouble, and they have lost their Romeo the night before they have to perform the scene. By doing so, Tristan never does kiss Rory again. On some level, was he quitting as Romeo in order to protect Rory from a kiss she never wanted to have, to save his own pride if nothing else?