Cats hiss at humans primarily as a form of communication to express discomfort, fear, stress, or annoyance. It is a defensive behavior signaling that the cat feels threatened, overwhelmed, or wants space. Common reasons for a cat hissing at a person include feeling startled, anxious due to unfamiliar situations or people, being overstimulated during petting or play, protecting territory or offspring, or pain and physical discomfort. Essentially, a hiss is a warning to back off to avoid escalation to more aggressive behavior. Paying attention to the context and the cat's body language can help understand the exact cause behind the hissing.