i believe it is just because as it has been mentioned, "human beings are not biologically inclined to mate for life. For LIFE is very important. Marriage is a union of two, which among other reasons, is created to achieve certain goals. So while these goals are common and are shared by both partners, everything is going smooth. Once they start having different goals, different directions to go, once they reach different level of personal development, they already don't match each other. It is known that once a 7-8 years every person goes through personal re-estimation of him/herself. it is also known that critical ‚"age" for marriage is 7-8 years, and most of divorces happen after 7-8 years of marriage. I would call it biological cycle. That's why in the west early marriages are not popular. Here is the example. The guy is 23 years old, and has graduated from the college, and got a sales position in the shop, hoping to become a shop manager very soon. The girl is 19 and has just finished high school. They fall in love, and get married. At that time the guy is on the higher social level than a girl, as he already earns money, and is planning to be promoted soon. Later, the girl turned to be ambitious and career oriented. In a next 7 years she did her Bachelor, and then her Masters degree, and got high post in the large firm. The guy hasn't got promoted, as it has been expected, and is still occupying the same position of sales person in the local little shop around the corner. She jumped into the high corporate environment, she started having new friends, she is making more money than her husband, she is getting attention of men of higher level, than her husband, who by the time got depressed. Their lives clearly took different paths. In 7 years they found themselves on a different social levels, with different frineds, with different priorities and plans for the future. Most likely it will lead to the affairs in the first place, and most likely the GUY is the one who will look for some other woman, who is more attentive and more understanding.. the girl also might get into the affair, for the reason mentioned earlier: now she attracts the guys from the higher class ..so now it's a time to change the partners.. for the next cycle..
this is just a sample, scenario could be very different, but the idea is the same: every 7-8 years the person re-evaluate his/her life and gets re-evaluated by others.. and very often at that time all previous connections/relations get broken and the new ones are established..
Human being is still inclined to mate, but NOT for a life.. There are still couples who could overcome these "biological crises".. but to be honest, i am convinced that they had to compromize a lot of stuff.. either both of the partners, or at least one of them.
Only very very few couples who naturally could live together in complete harmony ALL LIFE..