The unnamed force in this oft-quoted passage is “love,” true, but it is a specific sort of love: protectiveness. It need not contain affection, although affection, empathy, intimacy, romance, and gratitude are among the things that strengthen it. Snape uses this force to hide his protection of Harry from Voldemort, who would not be able to understand why anyone would protect a person they truly dislike. This is what powers Snape’s Occlumency.
