My current in laws are wonderful people. Sometimes they are just way too much to deal with - they are a big, loud, opinionated family - but they are such good people. I had mostly the same experience with my previous in laws (first marriage) too.
However.... I dated a man for five years whose family, especially his mother, just did not like me. She had extremely specific ideas about what kind of person her son should be with and I was not that person. Plus, she had no sense of boundaries or tact and would say the most outrageous things to me. She also tried to convince him to move to New York, knowing that I was not able to leave this area. We had a very hard time with each other. It was one of the factors that fed into us not taking our relationship to the next level.
There are a number of reasons that people could have issues with their in laws... most likely as many reasons as people who have in law issues. Sometimes people's personalities just clash. Sometimes there are addiction issues. Sometimes there are mental health issues. Sometimes the DIL or SIL is viewed as "just not good enough." Sometimes there are class or religious or racial issues. Sometimes, like my ex-boyfriend's mom, they are just focused on their own agenda. It might mystify you, but I suggest you enjoy your relationship with YOUR inlaws and try not to judge others' relationships with theirs.