The reasons why cats always scratch their nails on their owner’s pants:

1. Want to attract the owner’s attention. For example, if the cat wants its owner to play with it, or if it is thirsty or hungry, the cat will rub its owner's pants.

2. Expressing happy emotions. When a cat feels happy and satisfied, it will like to sharpen its claws. For example, when the owner just comes home from outside, the cat will be very happy and will grind its nails on the owner's pants.

3. Expressing dissatisfaction. When cats are unhappy or feel wronged in some way, they will also vent their frustration by grinding their nails. If the owner makes the cat unhappy, it will grind its nails on the owner.

4. The cat is stretching. When you find a cat grabbing its owner's pants and then stretching its back and hind legs, it means the cat is stretching.

5. Cats are declaring their sovereignty. There are sweat glands in the cat's foot pads, which can emit its own scent. Therefore, when a cat grinds its claws on its owner, the cat is marking the owner's scent, indicating that the owner belongs to it.