I believe that marriage is a failed institution because it is a tradition that is largely based upon long forgotten pre-historic ignorances.
Long before the advent of civilization and even before the advent of the spoken word there was a concept of the alpha male, although those who lived at the time probably didn't understand the concept of a concept and so alpha male wasn't anything particularly defined, but instead just an individual capable of displaying his physical superiority to others. I don't believe this to have been instinctual, but a behavior learned out of necessity and passed down through the generations.
As a means to project his power the alpha male not only beat his chest to display his physical capability, he also kept an alpha female by his side. Having this female by his side displayed to others that he was able to fend off all other suiters to claim her as his own, she was a symbol of his physical strength, a status symbol much like owning a Lamborghini or Bentley today.
As the world evolved packs became tribes and tribes became cities. With so many people around it was increasingly unlikely that they all knew one another, being as how the male would often find himself away from his woman it became necessary to display to others that this woman belonged to someone in order to frighten them away in the event they took an interest in the woman.
This is where marriage really evolved from the basic display of strength to a formal exchange of 'vows.' Around the same time laws were evolving and marriage was merely a means for a man to claim legal ownership over a woman, the exchange of items, such as rings, was a means to display to others that a particular woman already had a suitor who was willing to defend his property.
Mind you at the time woman really didn't have protection under any type of law which was generally part of the male's world. It was rather expected that a male would often take his woman by force, which is to say rape. If a male wanted to prevent his property from being raped he had to let the potential rapist know that she already had a male suitor who would be willing to avenge this rape, if she had no suitor then a male had no reason to fear taking the woman by force.
Over generations the concept of marriage evolved to what it is today, which is obviously different depending on the culture, but largely the original purpose remains the same. Marriage today is used as a means to establish ones status, although the status now goes both ways. The average person generally uses marriage as a means to establish their desirability while wealthier people also use it as a means to display their wealth and power. After all the term "trophy wife" is often used today and it means exactly as one would expect, the wife is a trophy and like any trophy a means to display ones superiority.
I find that, although marriage has evolved over generations, the ultimate goal of marriage remains unchanged from its origin. It remains the same as the beating of ones chest, it is a display of power and position. Marriage, neither in its inception nor as a concept today, has never been a representation of love. Marriage is a primitive concept developed by pre-historic apes, it means nothing.
P.S. I should clarify that I full well believe that people may find someone with whom they share a strong and meaningful connection, someone they desire to spend the rest of their lives with. I simply don't care for the idea that one must turn this connection into a legally binding agreement nor do I see the necessity to have this connection declared by some arbitrary individual or made 'official' through arbitrary rituals or traditions.
It's a choice like any other. The person or people you choose to spend the majority of your time with is up to you, call it whatever you want.