This point has been much more succinctly stated by Tolstoy in his book Anna Karenina, which begins:
Happy families are all alike; every unhappy family is unhappy in its own way.Jared Diamond turned this famous line into a full-blown principle in his book Guns, Germs and Steel to describe a system in which a deficiency in one or more elements dooms it to failure. But on the other hand, a successful system is one where every one of those failures is avoided.
Diamond uses the principle to illustrate why so few wild animals have been successfully domesticated: because the animal fails domestication on one or more elements. Domestication occurs not because of the presence of some positive attribute, but rather the lack of any negative ones. In other words, successfully domesticated animals are all alike (in the sense that they pass all the tests), unsuccesful animals are different in their own ways. Some are too skittish, some are too independent, some are too mean.
Diamond's usage aisde, Tolstoy's was illustrating how an interpersonal relationship, such as a family, can break down and be "unhappy" in any number of ways, but a happy family passes all the required tests. This isn't to say that happy families are perfect, or the same, but the necessary elements for happiness are all within acceptable range.
In modern dating the elements of communication, intimacy, respect, security, time, all need to be present to have a "happy" relationship. You can do very well at many of them, but fail at one of them and the system begins to falter. This is daunting enough without acknowledging that external forces such as finances, health, family pressures, and even death in a family, can undermine an otherwise successful relationship.
So, it seems that finding a partner, and having a happy relationship, is somewhat akin to domesticating a wild animal. Some of the typical factors that have ruined an animal's chance at domestication are picky diet, size, problems with breeding in captivity, nasty disposition, tendency to panic, and independence. Interestingly, I've found that if even one of these six elements fails while dating then the entire relationship veers dangerously into the realm of failure.
Have you ever tried dating someone who hated most types of food? It's hellishly annoying. Have you ever dated someone with the disposition of a mongoose? Or have you tried to build something with a lone, independent person who thinks they want to be in a relationship but really they have no business even trying?
But the sobering thing is that you can date someone who loves the same food as you, the same music, someone who is pleasant, who fits nicely next to you on the couch, someone who lights up a room, but if that person has the panic reflex of the North American deer, and bounds over the nearest fence every time something startles, the system will prove difficult to sustain. If that person can't communicate, or provide love or respect, if the system fails in one area, none of the other positive attributes can save it.
Faced with all of that, the quest is worth it. People are still out there trying to find other people. Go ahead, go to the park and sit on a bench and watch at the couples walk by. They're out there! For every hundreds of domesticated failures, there is one that has learned to play nice and provide. For every zebra and lion and their wild cousins, animals who have bit and mauled and carried off primitive man, there is the llama, the pack mule, and the chinchilla, animals which fail none of the required tests.
Happy relationships are all alike; but unhappy ones are unhappy in their own ways. For every successful relationship there are hundreds of attempts lying in the rubbish pile, smoldering like burning tires in a junkyard. Relationships that were tripped up by one or more failing attributes, or external attributes. Solid relationships full of excitement and fun but lacking honest communication; full of respect but lacking intimacy; full of stability but lacking time. The successful combine all the necessary requirements in the right way and make all the prior attempts worthwhile.