The reasons why cats’ claws keep opening and retracting are:

1. When a cat discovers something novel, it will tentatively touch the object with its claws.

2. When cats play, grab things, fight, etc. with people or other animals, they will first use their paws to test each other, and then wait for opportunities to move.

3. The cat’s claws may be injured or diseased. As long as the claws are exerted, there will be pain, and the cat will have this kind of stretching and retracting behavior.

4. When a cat encounters a threat or catches a mouse, the cat can use its claws to attack and defend.

5. Cats may extend and retract their claws when stepping on milk.