1. Selecting the wrong mate.
Most people get married before they truly know who they are marrying. They spend more time planning the wedding than they do their lives together. (In fact quite a few people don't know themselves!) They haven't done the introspective thinking to figure out what they really want in a mate for life. Even if they have, it's likely to change as they evolve.
What you think is perfect at 22 may not be so good at 32. Both people have to be growing in the same direction and want the same things (in the long run). If you want to go (right) and your mate wants to go (left) then you aren't "right" for each other.
Opposites may attract for the short term but "Like attracts Like" for the long term.
2. People realize life is short and no one enjoys settling.
This happens when people marry someone they see as having "potential". However often times it goes unfulfilled.
Once someone realizes they can't "change" their mate in order to get what they want then they are left with 2 options.
a. Stay in the marriage and learn to "live without".
b. Get out and find someone who already "is" what you want.
3. They got married for the wrong reasons!
(An ultimatum was given, a pregnancy occurred, was tired of being single, had a age deadline, all of their friends were married, just because they have been together for so much time….etc) “A marriage based upon (circumstances) rather than (love) is likely to fail."
4. The stigma is gone. Much like Bankruptsy...etc so many people have been divorced or filed bankruptsy that the majority of people don't view it as a negative reflection on the individual.
In all honesty I don't believe the majority of couples get married planning to get divorced. (It all goes back to selecting the "wrong mate") Not that there really is a "right" or "wrong" but more or less it's about "agree" and "disagree". Ultimately we're all looking for someone who agrees with us on the majority of things!
"When we change, our circumstances change"