Affinity. Or sometimes it means "I want to get into your pants." Or sometimes I suspect people say it just because they're in a relationship and the other person says it and they have no choice but to say it back.
Also, according Neale Donald Walsch, often "I love you very much" actually means "I trade you very much." I guess that's likely because, like Sean said a while ago, people don't get into relationships unless there's something in it for them.