Here's what science has to say:
Girl, we feel you. (Or guy. Guys can cry, too. And psychologists say that emotional control probably isn't good for men. So go ahead and let it out.)
You shouldn't feel shame about shedding tears of emotion. Weeping is part of what makes you human. Although other animals may yelp or whimper in pain or fear, and many creatures have tear ducts in their eyes to help flush out dirt and irritants, humans are the only species known to cry for emotional reasons. And scientists aren't really sure why.
One theory is that tears are a communication tool. Before they learn to speak, babies cry to get attention. They start out with tearless wails, but at around three or four months, they start to weep when upset as well. Evolutionary psychologists have argued that infants' tears are related to the distress vocalizations produced by other young animals: Crying conveys their need for parental care. It's also thought that a baby's crying has evolved to be especially evocative for parents — something few stressed-out, sleep-deprived parents of newborns would disagree with.
This theory would explain the loud, chaotic tantrums thrown by children when hurt or distressed. But what about adult emotional tears, which are usually much quieter? In those cases, crying could be a method of “conspecific communication” — a way of alerting sympathetic neighbors that something is wrong, without attracting the attention of a predator.
“It’s the biological equipment used by infants to maintain proximity to their caregivers,” Ad Vingerhoets, professor of social and behavioral sciences at Tilburg University in the Netherlands, told the Atlantic in 2013.
In other words, humans are social animals, and crying could work as a sort of relationship glue.
There's some data to back this up. A 2008 study led by psychologist Lauren Bylsma of the University of South Florida found that people were more likely to feel better after crying if they received social support during their tears. They also reported feeling catharsis when their weeping led to a resolution of the tear-inducing event, or helped give them a new understanding of what was wrong. By contrast, people who tried to hold back their tears, or said they were ashamed to cry, were less likely to experience catharsis.
In 2000, Vingerhoets led a broad review of the research on adult crying, and he came to some interesting conclusions. Adults were more likely to cry in the privacy of their homes, when they were alone or with just one other adult. They were more likely to cry for reasons related to rejection or separation than pain or injury. Many cried because they felt lonely, overwhelmed or powerless.
These findings put a wrinkle in the “crying as communication tool” theory, but they might support another theory of crying. Jay Efran, emeritus professor of psychology at Temple University, has noticed that humans are less likely to cry in the midst of a tense experience — the moment when they most need help from others. Instead, they cry after the tension has lifted. Think about a parent who loses their child at a grocery store, spends several stressful minutes scouring the aisles, and bursts into tears only after finally finding the kid playing among the candy bars.
Efran proposes a “two-stage theory of tears,” in which crying is our body's response to the rapid emotional shift that happens after the end of a stressful or emotionally charged episode. They are part of the parasympathetic nervous system's “rest and digest” mode, which takes place after the sympathetic nervous system's “fight or flight” mode has passed.
Chemists analyzing the composition of emotional tears — also known as psychic tears — have found some evidence that they may be healing. These types of tears contain several protein-based hormones, including prolactin, which is associated with breast-milk production in women and reproductive health in both sexes, and the neurotransmitter leucine enkephalin, a natural painkiller that gets released when the body is under stress.
A 2011 experiment also found that juvenile mouse tears contain a pheromone that protects them from unwanted mating by adults, and that adult men are less likely to rate women in photos as attractive after sniffing someone else's tears. The simplistic way to interpret this might be “tears are a turn off,” but the researchers say the real implications are more nuanced. It's possible that tears send a signal that the crier wants to be treated with compassion and care, as you'd treat a child.
“Certainly early in life, crying indicates that the organism is tense and overwhelmed — it's a signal to the caretaker that they need some help,” Efran told The Washington Post. “So in evolutionary terms, it's sort of efficient because it signals that help is needed and also indicates a system rehabilitation or recovery.”