Cats rub against humans primarily for social and instinctual reasons. They show affection and bond with their human companions through this behavior; it's their way of saying "I love you," "I missed you," or "Pet me". Cats have scent glands on their face, cheeks, chin, and body, and by rubbing against someone, they transfer their scent, effectively marking that person as "theirs" or part of their territory, which provides a sense of security and signals to other cats. This rubbing behavior also releases endorphins that feel good to them and reinforces social bonds within their group, including with humans they trust and accept as part of their social colony.
In addition, cats use rubbing as a friendly greeting or to gather information about new people or environments, signifying comfort and trust. Some cats also rub as a way to initiate play or seek attention or food.
Overall, when a cat rubs against a person, it is an affectionate, communicative, and territorial behavior indicating trust, bonding, and social connection. It shows the cat considers the human part of their safe social group or "colony".
