Parents probs

I have been dating my bf for about 4 years and since day one I have always felt like my parents have some towards him. He said not done one thing wrong. He comes over and is very respectful and nice. But I don’t get why my parents are like that. They talk shit about him every chance they get. They are just so fake sometimes. I hate it so much bc my bf doesn’t deserve that. I just don’t get why my parents are like that and how to deal with stopping the bullshit.