Full, red lips are considered sensual - and are therefore a sought-after beauty treatment.

Newly in love couples can kiss for hours. Close friends give each other a kiss on the cheek as a greeting, parents give their children a loving goodnight kiss. Kissing can be passionate or tender, but it always shows that you're close.

It's no wonder, then, that lips are so important to us—and many people resort to them for beauty. Lip fillers are trending, partly thanks to social media, where young women find role models. Is this noticeable when kissing? And when is it too much? Experts offer answers on World Kissing Day on July 6th:

"Full, beautiful, sensual lips represent youth, fertility, and even sexual attractiveness," says Helge Jens, a specialist in plastic and aesthetic surgery. "Rosy, full lips simply appear vital, while thin, dry, colorless, or wrinkled lips tend to symbolize illness and aging."

From the perspective of Berlin psychotherapist Wolfgang Krüger, this is also true: "The lips essentially play a major role in our emotional life. Kissing is the best and most intimate way to create closeness." It's even more intimate and sensual than sex, says the expert, who has just published a book on the subject. When kissing, faces come very close, and you feel an inner connection with the other person. "You also notice this when relationships become more difficult. The first thing to suffer is romantic kissing."

That's a difficult question to answer. A look at the statistics from the German Society for Aesthetic Plastic Surgery (DGÄPC) shows that lip corrections ranked only 15th among cosmetic procedures last year. In 2014, they were still in seventh place. Looking around on the street, however, one gets the impression that lip augmentation is significantly more common.

DGÄPC President Helge Jens also confirms this impression. "I think that lip augmentation is largely not performed in specialist plastic surgery practices, but rather by beauticians or in specialized beauty institutes. Even alternative practitioners are allowed to do it these days," he says. Hyaluronic acid is usually injected into the lips, and virtually anyone can do it, since it doesn't require a prescription.

"The vast majority are young women who want to emulate a certain beauty ideal," says Jens. Their source of inspiration is often examples from social media. "Patients actually come to the practice with printed pictures or cell phone photos and say they'd like it exactly the same." A trend he finds problematic, since the lips also have to match the face.

Christiane Bayerl, Director of the Department of Dermatology and Allergology at Helios Dr. Horst Schmidt Clinics in Wiesbaden, primarily sees women who have gone through menopause. "Lips become thinner with age." Thin lips can be perceived as pinched.

Jens and Bayerl say they use less than one milliliter of hyaluronic acid for lip augmentation. Bayerl says she sometimes sends women home who already have inflatable lips but still want more. This poses the risk of blocking blood vessels, says the expert from the German Dermatological Society.

Yes, and not just for heavily augmented lips. "Just because most treatments work doesn't mean they're harmless," says Jens. "You need to have a precise knowledge of anatomy to know where to inject hyaluronic acid to achieve the desired effect."

According to Bayerl, common side effects include bruising if the injection hits a small blood vessel. "It can also happen that it becomes asymmetrical and needs to be corrected during a follow-up treatment." There's also the risk of infections and—in very rare cases—vascular occlusions, which must be treated to avoid serious consequences.

That depends on how much lip augmentation has been done. If it's minimal, you probably won't notice a difference, says Bayerl. "If it's very pronounced, it will affect sensitivity." The reason: Lips contain nerve endings that make them sensitive to heat, cold, and touch. "These nerve endings don't become larger with the augmentation; they need to be spread over a larger surface area," explains Jens.

Psychotherapist Wolfgang Krüger doesn't think that's the case. More important, he believes, is the way a person lives. "I think you can tell whether a person has a sensual willingness to kiss." If someone has a lot of problems, is tense, and reserved, it will eventually show in their facial features. "The person in question appears to have thin lips." While lip surgery may change the appearance, the lifestyle remains the same.

Humans are the only species on Earth that kisses. Why we developed this behavior cannot be definitively answered from a scientific perspective, says Wolfgang Enard, an expert in evolutionary anthropology at the Ludwig Maximilian University of Munich. "One prominent idea is that kissing arose from the passing of food, i.e., pre-chewing." There is also the theory that kissing serves to exchange bacteria and thereby strengthen the immune system.

However, Enard considers another theory more plausible, which, in his view, also explains why humans kiss, unlike their close relatives: "In chimpanzees, social interaction largely revolves around grooming. They groom each other." In humans, however, the fur has disappeared, and the kiss has evolved as a remnant of grooming. Whether a romantic kiss or a friendly kiss on the cheek as a greeting, "it is always a strong social signal."