He became completely unhinged at that moment. He saw only red and became enraged. Somehow his happiness gland turned into some sort of mega-fury gland. It's hard to explain what happened next.
The National Guard's Emergency Cheeseburger Department was called in to bring Happycat several hundred burgers in order to resolve the incident peacefully. Happycat's head decreased in size and returned to his body. He was once again happy and all was right. After some more study, scientists discovered that the proteins found in beef and cheese stimulated that happiness gland. If he goes too long without them, it can severely disrupt his brain chemistry.