And yet it can be troublingly hard from the outside, at least to understand why.

Even though the matter is one of the most urgent for anyone reflecting on relationships and their chances of succeeding at them.

But we can know for sure is the success in love isn't about luck.

Couples who make it will, on frequent occasions, have looked very much as if they wouldn't.

They will have despaired, slammed the door, packed a suitcase and walked to the station.

But they won't have boarded the train, not because they weren't sorely tempted, but because at the last minute they remembered something key about human beings:

that everyone is in the end pretty much as awful as everyone else.

They didn't allow themselves to get overly idealistic about strangers.

They understood that the difficulties they were facing would be those that would necessarily unfold with pretty much anyone they could be with, in one form or another.

They accepted that we are all intrinsically flawed, out-of-control disturbed creatures, mired in madness, error and confusion, and generally worthy more of compassion than fury.

They lost the will to believe in that most destructive of illusions: flawless love.

Those who wind up sticking around are those who can suppose, after a three-day-long argument followed by a week long sulk,

that neither their partner nor they are ultimately especially dreadful, only averagely and typically so.

They become each other's best friend rather than upbraiding, angry angel.

They recognize that they are trying to do something extremely complicated together, and block their ears to society's malicious suggestion that it might ever be easy.