Cats raise their butts when you pet them primarily because it feels good to them. The base of a cat's tail has highly concentrated nerve endings that are very sensitive to touch. When you pet this area, it creates pleasurable sensations for the cat, causing them to lift their rear end to maximize that feeling. This automatic response to enjoyable petting is similar to how humans might lean into a good back massage. Additionally, this behavior is a sign of trust and social bonding. By raising their hindquarters and exposing a vulnerable part of their body, cats show they feel safe and comfortable with you. It's a way for them to communicate friendliness and allow you to continue petting in a spot they particularly enjoy. Other reasons for this behavior include:

• Bringing the "good spot" closer to your hand for more effective petting.
• An instinctive holdover from kittenhood when raising their rear helped mothers with grooming.
• A form of feline communication related to scent glands near the tail, which cats use to mark territory and identify each other.

In females, especially if unspayed, frequent butt raising can sometimes indicate they are in heat, as it is part of mating behavior. Overall, when your cat raises its butt during petting, it is a positive sign indicating pleasure, trust, and comfort with you.