This question is a little vague as he does this a few times in the books for various reasons. One time is because her father is about to walk in to her room. Edward is not supposed to be in her room and he doesn't want to get caught by Bella's father. Another time is when they were kissing and Edward needs a break. He gets wrapped up in the kiss and is afraid he will hurt her, so he takes a bit of a break before things go too far.