Why do we say "Beauty is in the eye of the beholder"?

Meaning:

Beauty exists in the mind that observes it. This expression is often used to describe a person or thing whose appearance does not match the commonly accepted standards of beauty.

Background:

Phrases.org.uk says that the first known reference to the expression is found in Ancient Greece around the 3rd century, BC; but it does not provide a reference source. However, there are plenty of variations to be found before the late 19th century, which is when the modern-day version of the expression is believed to have made its entrance.

For example, English playwright John Lyly's 1580 play, Euphues and his England:

"...as neere is Fancie to Beautie, as the pricke to the Rose, as the stalke to the rynde, as the earth to the roote."

or Shakespeare's Love's Labours Lost (1588):

Good Lord Boyet, my beauty, though but mean,
Needs not the painted flourish of your praise:
Beauty is bought by judgement of the eye,
Not utter'd by base sale of chapmen's tongues

or Benjamin Franklin, who put a neat spin on things in Poor Richard's Almanack (1741):

Beauty, like supreme dominion
Is but supported by opinion

Most sources attribute the first use of the modern-day expression to Margaret Wolfe Hungerford (née Hamilton) who wrote a number of books under the pseudonym of "The Duchess," and, in her 1878 work Molly Bawn, wrote "beauty is in the eye of the beholder."

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