What doesn't zero kiss yuki from vampire knight?

When you say what, i asume you mean why. Zero is a Vampire when he finds out he loves Yuki, so he doesn't want her to know he loves her. let me explain, Yuki has been open to giving Zero her blood, so Zero thinks if Yuki knew he loved her, Yuki wouldn't be careful when Zero does drink her blood. Zero only didn't kiss Yuki because he wanted to keep her safe. Hope that made sense!