What does "no love lost" mean and where does it come from?

I have trouble with the idiom "no love lost". I understand that it is used when people are at odds or don't get along, but I don't understand why. Interpreted literally it sounds like there should be plenty of love, but it seems to mean the opposite.

Where does this expression come from? And is there a different interpretation that better explains its meaning?

Searching Google books, I find that what the phrase originally meant in the 17th and 18th centuries was that "A loves B just as much as B loves A"; the amount of love is balanced, so there is no love lost. In other words, unrequited love was considered to be "lost". This could be used to say they both love each other equally, or they both hate each other equally. The idiom has now come to mean only the second possibility.

From this 1754 English-Danish dictionary we have a translation:

ieg elsker dig ikke meere end du elsker mig
(literally: I love you no more than you love me)

(1712):

You must know, Sir, I love Prudence, my Lady Laycock's Woman, and I believe there's no Love lost between us, nor do I know how soon we may exchange our Persons for better and for worse.

(1645):

He hates the Council here, and I find plainly there is no love lost; they fear he will seize on the Prince, and he, that they will take him.

John Dryden (1712):

— By Bottle and by Butt I love thee. In witness whereof I drink soundly.
— Your Grace shall find there's no love lost. For I will pledge you soundly.

There's also a translation of the expression in a 1732 Irish-English dictionary, but I don't know Gaelic.

If two people love each other, then fall out (because of an argument or other reason), then there was love lost between them. But if two people don't care much for each other, then have a falling out, then there really was no love lost between them.

Interestingly, when it was originated in the 1500s, until about 1800, it could indicate either extreme love or extreme hate.

Extreme love (the image is of love shared in a common vessel; when affection was mutual, none of the love in the vessel was lost):

• Sore sicke he was, and like to dye,
  No helpe his life could save;
  His wife by him as sicke did lye,
  And both possest one grave.
  No love between these two was lost,
  Each was to other kinde;
  In love they liv'd, in love they dyed,
  And left two babes behinde." - Reliques of Ancient English Poetry, 1765
  ^{_{You can tell it's from some time ago; the two innocent children die in the woods!}}

Here, Manville, a gentleman, loves a peasants daughter, Em. Em speaks of him:

• And never could I see a man, methought,
  That equaled Manville in my partial eye.
  Nor was there any love between us lost,
  But that I held the same in high regard, . - Faire Em, (a fraudulent Shakespeare) - Act V, Sc. I (1592)

Extreme ill-will

"There's no love lost," quote Sancho, "for she speaks ill of me too when she list." - Don Quixote. 1620 translation

Today, however, the term signifies ill will exclusively. If there is no love lost between two people, they have a strong enmity towards or hate for the other and make no effort to conceal it.

He needs her appearance of moral integrity, and she needs his iron to end all argument about her unity and purity. It is a marriage of convenience, a strained relationship, with no love lost between them. For both, the hope of world dominion is worth the tension.

An expanded version, "no love lost and no love found," probably makes it easier to understand the modern meaning of the phrase when looking at the literal interpretation.

I think it would mean "Don't assume that any love is missing just because the two parties hate each other... there was never any love there to begin with."