While valentine notes today tend to stress caring and warmth, love letters from ancient Rome often highlighted the wrenching, painful side of romance, historians say.

Valentine's Day itself didn't yet exist in ancient Rome, but men still wrote love poems about their sweethearts — often married women, and sometimes men. But where modern declarations of love often involve flattery and gratitude, the ancient Romans wrote more about pain.

Unlike what you see in contemporary stores where we have valentines that are all clouds and dreamy and romantic, the Romans had a very different kind of take on love," said Barbara Gold, a professor of classics at Hamilton College in Clinton, N.Y. "It's not something that is a good feeling usually; it's something that torments you."

She described ancient love poems from about the first century B.C. to around A.D. first century that call love a plague, accuse love of making the writer see double and causing his tongue to swell up.

"You would never go out today and find a valentine that says 'You're like a plague, you set my bone marrow on fire,'" Gold told LiveScience.

In ancient Rome ideas of romantic love were very different — most people never expected to love their spouse.

"Marriages were arranged, and all about wealth and status and power and keeping the family line going," Gold said. "There never would have been a marriage for the Romans that was based on any kind of erotic sexual attraction."

The love poems were all written by men, and were mostly directed at women they were having affairs with. Though the identities of the lovers in the poems are often hidden by pseudonyms, in some cases they are known to have been married, aristocratic ladies.

Gold said she suspects the dark tone of many men's love poems had to do with the sexual dynamics of the culture at the time.

"It's all about how they viewed women – women are a torment, women are a plague," she said. "I think it's because men are terrified of the power that women have, and they project those feelings."

Ironically, it's the men who had more power in ancient Roman society. But any force that made them feel less in control and less powerful — such as a woman giving or withholding her love — could be very threatening. Gold speculated.

The change in today's expressions of love might have to do with the different culture we live in.

"The way our society is now, there's a lot better chance of people coming into relationships as equals," Gold said. "We're not living under the same sort of social constraints as the Romans were."