Since he's the one who was telling her not to get attached, he can't say anything against her fear of being hurt. That was before, though—before he realized that he was attached, and he's selfish and not afraid of potential future pain.

But that alone doesn't justify doing something rash like kissing her right now. She's confused and unhappy and he has no real desire to worsen those emotions in her.

"Help me understand." Because he doesn't and he would like to.