Seth saw how funny her shocked expression was and couldn't help but look inside her eyes. He leaned closer slowly and joined his lips on hers.
Elyra would never expect something like this happening. That is why even though she saw Seth approaching, she didn't move from the place she was.
In a single second, her shock turned into confusion and she even forgot to move her head away. That actually made her experience a longer kiss than it should have been.
Seth originally planned to simply give her a peck on the lips, since he was interested on her. Elyra had a very mature body, what have been enticing him constantly whenever he was in her presence. However, he was always pushing those feelings aside and working with his main head. But after noticing her worry for him, he couldn't help but let his feelings take control and he suddenly started to push things forward.