Mother-in-law always speaks ill of her daughter-in-law and complains in front of relatives and friends. What's her psychology?

I don't regard my daughter-in-law as one of my own, but I don't like her. I don't think anything she does can make me happy. So, I made irresponsible remarks in front of outsiders and blindly belittled my daughter-in-law. The relationship between mother-in-law and daughter-in-law has been a big problem since ancient times. If you meet a kind-hearted mother-in-law, you will naturally be much happier. If you encounter insolence, you can only admit that you are unlucky.