The more of my friends' relationships I see fall by the wayside, the more I can't help wondering if half the people in the world aren't some sort of monster when they're at home?

You never truly know what goes on between a couple behind closed doors until they break up or divorce, then all the smelly old bones clatter out of the cupboard, sharpened into arrowheads to wound the other party.

He beat her, she cheated on him with his best friend. She put them into debt spending money like a Hilton, he snorted away the family home with a gaggle of prostitutes. She was a degenerate gambler, he talked to her like she was an animal.

It seems many couples are on their best behaviour when out in public, keeping up appearances until they can get home and he becomes a reclusive slob, she belittles him, or they wheel out some other combination of Jerry Springer pathologies.

It's neither a male nor a female thing - I've heard horror stories about husbands and wives, boyfriends and girlfriends alike. The common element is that some people - in fact, lots of people - reserve the very worst of their personality and behaviour for the person they claim to love most.

An old friend of mine recently lost custody of her children because of the manipulations of her husband, who first egged her on into cocaine addiction, then alcoholism, using a frighteningly calculated system of emotional abuse.

A male acquaintance weathered three years of litigation after his wife maliciously accused him of molesting their three-year-old daughter so she could gain sole guardianship of the child.

Both are extreme examples to be sure, but then you have to work hard to stand out in the sea of stories I've heard about partners of both sexes' laziness, rudeness, contempt, selfishness, violence and deceit - couples who at the time claimed to be "in love" or just going through a "rough patch".

The almost invisible line between love and hatred has been written about for millennia and it's no surprise the greatest of human passions can also inspire the cruellest of behaviours - but I still have to ask why.

Why would you speak to your partner in a way you wouldn't dream of doing to a friend or even a stranger? Why do so many people stop trying to control their tempers or their moods as soon as the front door closes and they're alone with their one and only?

I'd even cut someone half a break if that's the way they were all the time - but so many people just present a palatable and polite face in public or during the first three months of a relationship - proof positive they can successfully monitor and curb their crappy behaviour.

Relationships are tough, to be sure. They're another pair of eyes watching the movie of your life, and those eyes can often act as a mirror, reflecting at us the best and worst of our natures.

However, instead of seeing this as an opportunity for compromise and growth, many people view it as the time to attack, to shore up the fortress of their flaws by undermining the defences of the person they're most intimate with.

Why? Like most poor behaviour, I'd gauge it comes down to self-respect, or the lack of it.

When you think little of yourself, I imagine it's easy to despise a person who has so few options that they've been desperate enough to fall in love with you.

Sam de Brito's latest novel Hello Darkness is in bookstores now. You can follow him on Twitter here. His email address is here.