Today, I worked on a kissing scene first, followed by several other scenes. The other scenes went much faster.
Kissing scenes can be difficult because the characters are extremely vulnerable to each other. (Confession scenes also take me a long time to write, for the same reason.) Even if it's a comedic kissing scene, the characters are still vulnerable (in a way, I would argue, more vulnerable!). Also, in real life we have a natural inclination to look away from people who are kissing, to give them privacy. The writer, on the other hand, must stare at the characters in her head and describe their experience in enough detail to bring it alive. The first few times I ever wrote kissing scenes, I squirmed and blushed my way through. Then I would go back and reread and think, "Wow, I thought this was so intense as I wrote it, but it's really mild--you can hardly tell what's going on." It can take a long time for a writer to drop her defenses and just write about the kiss already!
Every kiss is different, of course--depending on the degrees of interest and experience the two characters bring to the event, and depending on how they feel about themselves and each other at the time. One nice thing about YA is that it often gets to incorporate a first-kiss scene (which is less common in adult literature). And people's reactions to a first kiss can vary so much:
"Oh, so that's what it's like? What's the big deal?"
"I am so glad he finally kissed me!"
"Was I doing that right?"
"So this is what everyone's raving about!"
"I suppose it must get better."
"I wish it had been [insert name] instead."
"When can we do that again?"
I try to capture the experience honestly, whether the characters' reactions are positive or negative: the bumped noses, the insecurities, the surprises, the joy. How characters handle these situations is as telling as how they handle every other experience in their lives: it's part of characterization. And a kiss is an action, a plot point, that can change the whole course of the story.