Can you please explain to me what this saying means? I've heard it many times and I just don't get it! It just doesn't seem to make sense to me... thanks in advance.
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Some internet quotes:

'The cliché, "All is fair in love and war," implies that people can suspend the law or the rules in special circumstances.'

' "All is fair in love and war" - Behavior that is unpleasant or not fair is acceptable during an argument or competition.'

'ALL'S FAIR in love and war, we hear at a tender age. Though this is tempered by schoolboy concepts of fair play and never hit a man when he's down. Fair play is reasonable if you don't mean to win at any cost and the other guy doesn't mean to kill you, but all that goes by the board in any genuine confrontation. Juvenile tussles are one thing; a real fight is definitely something else. And so is real love and real hatred and anything else that's real. You don't learn to play poker by wagering matchsticks. Vae victis, or, as we say in English: Losers weepers.

"All's fair in love and war," quotes Frank Farleigh (1850, by Francis Edward Smedley, 1818 - 1864) and is the popular paraphrase of "Love and War are the same thing, and stratagems and polity are as allowable in the one as in the other." - Miguel de Cervantes (1547 - 1616), Don Quixote (1605 -1615)'
Hello Guest, hello MisterM

I've mostly heard 'AFiL&W' where the context is 'love', rather than war. When applied to 'war', it has almost the air of a novelty.

The usual context would be something like:

'Tarquin, by a fiendish subterfuge, has engineered the absence of gullible Pozzo for 3 days; during which time he seduces Dorothea, of whom gullible Pozzo is enamoured. AFiL&W.'

'Sultry Colomba informs Gigi that the seemingly innocent Nicoletta has been found in compromising circumstances with indefatigable Charlie Holmes. It is a lie, of course; but Gigi, furious, breaks off his engagement to Nicoletta and sweeps sultry Colomba off for a rampant weekend in Monte Carlo. AFiL&W.'

It seems to me that the phrase is in effect 'all's fair in war' humorously extended to include 'love'.

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There are no rules or restrictions in these two endevours. They are both things that bring the best and the worst out of people. They are both battles of the heart, and battles of the soul. "All is Fair", they cannot be controlled, they cannot be contained.
It means at least in my opinion that there are no rules when it comes to love and there are no rules when it comes to war which means everything you do is fair.
You have to pay special attention to 'All' in "All is Fair in Love and War." 'All' in this context is meant to express that nothing is out of bounds when it comes to love and war. Everything is fair game. You might misinterpret this phrase to mean that "love and war is fair," this is likely a common misconception. 
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It's really about winning.  If you are in love with someone, you will do whatever it takes to win that person's love (if possible).  You'll spend countless hours strategizing how to win his/her heart and ultimately some of the choices that you may make may border on bizarre, outlandish, deceitful and maybe even illegal.  Emotionally its hard to resist the urges to do whatever it takes.
Think the difference between playing a game where the outcome is simply a learning experience (win / lose or draw) a war which is a fight to the death or the pursuit of someone you are totally in love with.  There is no such thing as a fair fight.  The woman of your dreams should be fought for.  In fact she may even respect and love you more because it shows that you are a protector of the things you love. 
 I was reading your response and I think what you speak of is mostly true, still trying to decide. See, I believe like you, I think(here is my standing point). Love and War are things to be compared and contrasted. Love is something that has no rules and people feel like with the two, you fight to win. There is always a point at which in the two you shall stop. War should and does have rules, it can't be attacked the way people attack love. Love has no rules to it, so that is why it works or it does not. Love is something that isn't fair as in war. People will and won't fight for the two, so where do you decide what in fact is or isn't fair? Is all fair in love and war?
you know that everything has it own rules ...
well there are no rules when it comes to love & war :
ALL IS FAIR Emotion: smile
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