Why do some have problems with the mother in law?

I just don't get how some people can have really bad problems with their in-laws, I don't mean to sound like a rude bragging bitch to those who do have bad in laws. I've just never had a problem. With it and if they didn't like me I said whatever and just went about my day and always tried to make them like me by helping out or cleaning the house while they went out to shop and saw something needed to be done so I cleaned it. Now my current boyfriend his parents don't speak English very well but they know the basics and even before I was trying to learn their language they like me, his whole family likes me. (they speak khmer, which is a cambodian language) and I'm sorry for those who have bad in laws. What makes them not like you or anything that you can tell me so I understand? My grandma on my dad's side didn't like my mom and my mom never cared she didn't like my grandma either. Is that the way it works you both don't like each other? Sorry again I'm not trying to sound like a bitch I'm just trying to understand it. My family and I have the attitude of "I dont really care what you think of me." so it's harder for me to understand since i have that mind frame