Failure in marriage is for the same reasons of failure in anything. Lack of knowledge and preparedness. You might think we would have full knowledge based on our parents example or lack thereof, causing us to seek out and prepare ourselves. Not the case often, is it?
As a human enters their adolescent stage of development, we all know and have experienced the distraction created by the chemical changes in our bodies and minds. We make a significant amount of decisions on base instinct during this time and have little, or not strong enough, guidance within ourselves or from others to overcome what is leading up to and often includes life-long choices.
Once we begin to "calm," somewhat, as things biologically level out (hopefully), though this can and does take additional years for many, we are still not knowledgeable and prepared to meet the expectations of our society and/or culture. Whether in personal or professional life. Leaving us caught between making decisions based on recent instinctual experiences and what we need to know to significantly impact a portion or the span of our further existence.
Past generations, majorly, have denied their individuality, in favor of meeting the expectation of society/culture/family/spouse. This is safe, especially without knowledge for what else could be achieved or evolved into. I do not mean this generally, as there are obvious exceptions, though reportedly rarer than the norm.
As humans have evolved in recent generations as more capable, knowledgeable and experienced individuals, those "striking this iron" earlier are waiting to "pair up" later in life. Unfortunately, there are still entirely too many individuals unguided, lacking knowledge and preparedness, who would benefit from being taught to know their selves better in mind and body from early adolescence on.
I was married at nineteen. 6.5 years later and two children later, our marriage failed as we grew as individuals and apart, exactly for the reasons I listed above.
After our children were grown and on their own, I remarried and am not dealing with the same lack of clarity, my first wife and I experienced. My son and daughter, ages 28 and 25 respectfully are in no hurry to marry, as they realize and have learned, they tell me, from my guidance, also based on what I have learned and mentioned above.