Cats rub their heads on people primarily to mark them with their scent. Their heads have scent glands that release pheromones when they rub against something or someone, signaling ownership or marking territory. This behavior is a way of saying "you belong to me" and also shows affection and social bonding, similar to a friendly greeting. Cats also gather information about the person or environment through scent during this behavior. Additionally, head rubbing can be a way to get attention or show trust and comfort with the person. Overall, this head rubbing is a mix of territorial marking, affection, and communication that strengthens the bond between the cat and its human.