Our ancestors didn't have many choices, before there was no internet, pornography, TV and other access that made them have varying standards of beauty. So the standard of beauty according to our ancestors was based on the area they lived in. compare it to today, with access at your fingertips. We have very broad fantasies and beauty standards, so nowadays it is very difficult to commit to marriage and respect the partner we choose in marriage, only strong people can. but yes, it depends on the person too. in my country the divorce rate is very low.
Also, our ancestors didn't have the alternative to divorce, with rare exceptions, because it was against the morals and laws during the ancient times. To be a divorced individual in past times was a curse, especially for women, who totally lost their values inside the society, so they prefered to remain married, even if unhappy, as after measuring the pros and cons, they reached to the conclusion it was more benefical to them to stay together, at least formally, in front of the society.
Nowadays the society doesn't see divorced individuals in a negative way anymore, so people don't feel socially prejudiced when deciding to abandon their partners. There isn't a punishment by doing so anymore. It's quite the opposite, they see this as a new opportunity to find someone new and better than the previous one. That is what we call "liquid love". They don't last for long, they aren't solid, they are volatile and in other cases highly artificial.
People just use each other and once they get bored, they go to the next one. It's the individuality trait of the "self" exercising full control over the individual, and demanding to be served by the world outside incessantly, without giving anything back in counterpart.