Severus noticed Lily moving closer, how could he not notice? In his mind a battle was raging between his desire to kiss Lily and the knowledge that it was not proper and that it was probably going to end with a broken heart in the near future. His broken heart. He knew Lily would never intentionally hurt him but Severus also knew that Potter would.
"I just know one dance, because you taught me." Severus blushed, not just because of the compliment but also because of how his mind showed him how easy it would be to lean in and kiss Lily.
He knew he had a few options now, act clueless, kiss Lily or talk to her about what he was feeling. For the first it was too late already and the last was going to be so very awkward, after all he wasn't much of a talker. That left kissing Lily, he looked at her lips and without thinking about it he licked his own. He leaned in, for a moment fearing that kissing Lily would be the thing that send her into Potter's arms, then he pressed his lips chastely against Lily's, his very first kiss ever.