Same with tears of joy (which many musicians have documented in song), or whatever it is that happens while watching videos of soldiers reuniting with their families: It's a happy, ecstatic moment, and yet we (specifically, me) are reduced to a puddle of tears.
Negative and aggressive responses to positive emotions is something Yale psychologist Oriana Aragon realized that science hadn't really taken the time to explain. She became inspired to examine such "dimorphous expressions" of positive emotions, as she describes it, after hearing actress Leslie Bibb describe this impulse to Conan O'Brien, which you can watch in the clip below. Essentially, you have two incongruous responses stemming from the very same emotion.
So Aragon and other researchers studied these emotions and responses, the results of which are to be published in a forthcoming issue of Psychological Science.
Initially, the researches asked 143 participants questions including whether they cry when seeing loved ones reunited or while watching the happiest moments of movies, and whether when holding an "extremely cute baby, I have the urge to squeeze his or her little fat legs."
They found that those people who express "negative" or aggressive responses in one scenario were likely to do it in another; for instance, those who cry at their kid's graduation were more likely to want to also want to pinch a baby's cheeks. And no, not everyone has such reactions, which take place "when people are overwhelmed with emotions," Aragon said.
"It took me a long time, a lot of experiments and a lot of work, to say: yes, people are actually feeling positive [emotions] but expressing negative [responses], and do so across a variety of situations," Aragon said.
Then, nearly 300 participants were shown pictures of babies, which altered to make them look more or less cute. The cuter babies were viewed more positively and elicited responses such as "I want to pinch those cheeks!" or telling the baby, through gritted teeth, "I want to eat you up!"
Researchers measured people's feelings immediately after viewing those photos and then five minutes later. They found that people who had aggressive or negative reactions to positive emotions actually recovered better from their emotional highs and got closer to an emotional equilibrium than people who had no interest in, say, pinching cheeks.
In other words, Aragon said, those inherently negative reactions -- tears of joy, for instance -- may help people calm down from their emotional highs.
"We really want emotional homeostasis," Aragon said. "We want a happy, middle spot. Extreme is not good. It's hard on our bodies."
That may be one reason why some humans exhibit this behavior. There could be others -- but, yes, such uncontrollable feelings can be trying on the body. Just watch this video of Kristen Bell's emotional meltdown when her husband surprised her with a sloth for her birthday:
And this concept is likely more universal rather than strictly cultural. Aragon asked a series of language professors if different cultures had expressions or words for such reactions to babies, along the lines of "you're so cute, I want to eat you up with a spoon!" And indeed, there were countless examples. In Tagalog, "gigil" refers to "gritting of teeth and the urge to pinch or squeeze," the researchers write.
And here's one more: in Farsi, it's not uncommon to hear an adult tell a cute baby: "I want to eat your liver!" While that may sound aggressive, it probably helps me calm down.