* He's interested in you. This is the most likely explanation, especially if he's been flirting with you or showing other signs of interest. By asking you to slow dance with him, he's taking the opportunity to get closer to you and see if there's a spark.
* He's just being friendly. It's also possible that the guy is just being friendly and doesn't have any romantic intentions. He may simply enjoy dancing and want to share the experience with you.
* He's trying to make you jealous. If you're already in a relationship, the guy may be asking you to slow dance with him in order to make your partner jealous. This is a manipulative and immature tactic, and it's not worth your time.
Ultimately, the only way to know for sure what the guy means is to ask him. If you're interested in him, tell him how you feel. If you're not interested, be polite and let him down gently.