Reasons why cats scratch people:
1. Cats are very alert and sensitive. When you try to tease them with your hand, they will naturally try to scratch you.
2. If the owner scolds or hits the cat, frightening it, the cat may retaliate by scratching or biting the owner to express its emotions. 
3. If your cat's claws are too long, it needs to find a place to sharpen them. It may scratch not only people but also the sofa at home. You can prepare a scratching post for your cat.
4. If the owner touches a cat's sensitive areas and makes it uncomfortable, the cat will resist and scratch.
5. If you tease your cat or move its food bowl while it is eating, it will think that you are taking its food and will attack.
