Why do married couples often hate their in-laws?

Not hate, more like annoyance. Many get the feeling inlaws don't think their good enough for their sister, brother, son, or daughter.

The first answer is excellent. In my case, I really did not like my ex's father at all. He beat his tiny wife for 20 years, and my ex thought that was an OK thing to do to me. To say that's wrong is putting it mildly. That's why he's an ex. I think I wanted to divorce his father too! lol And it's true, many times in-laws truly think their son/daughter has married the wrong person. My ex-mother-in-law was very sweet, and on more than one occasion I had to defend her - and not just with words. Her life was miserable. I wasn't about to let her husband and my husband make mine miserable too. My husband now of 34 years is 18 years older than I and we married when I was 31 and he 49. His parents were gone by the time he came into my life, but as his father was a good 'ole boy, he would have hated me as in his eyes I was a "damn yankee." Thank heavens my husband did not share that attitude. We're very happy and no in-law problems. :D

Well me and my husband not hate our in laws. My mother in law and father in law has been good to me and my husbands grandmother and great grandmother were very good to me and loved me very much.

My mother just has always give my husband a hard time but they get along of the most part. My father likes my husband and they have never really had a bad word.

Most women do. Most men know they can't afford such a situation since they are not good at handling drama. So they would be civil to their in laws .. at least in public.

Women do it because they know they can get away with it and don't have to put an iota of effort at reconciliation.

There are four steps to life in it self........first you are born.......then you live........you procreate .......and then you die.......with the part where you procreate comes a part that most parents do not like to accept and that is the fact that their kids have a life of their own now and they want things to remain the same as when they were just their kids............it is hard for most people to let go and allow their children to make mistakes just as they did once............I had wonderful parents that adored my husband to the point of calling me to my face "HIS little woman" I loved every minute of it too >>>:)

That is a misconception. You have a brain give the inlaws a chance, you might be surprised, they might be great people.