It's human nature, much like it is human nature how most people hate freedom.
People didn't live very long up until 100 years ago or so, therefore being married until death do you part, was more likely than not.
Now, stick two people together for 20-30-40 years or more, and before long, they begin to loathe the very sight, smell and voice of the other person.
The sad part is, that loathing is usually one sided, one or the other will still look at their partner, who despises them, with love and affection and desire.