Eyes communicate intent
Fair warning: this answer got long! The gist of it is that eyes are an important and useful indicator of intent due to a long history of biological and cultural evolution. Hence, nonconformity with societal expectations regarding eye contact with other people usually leads them to think something is amiss.
The major points of this answer are:
- Animals react to eye contact in various ways.
- Humans are biologically hard-wired to respond to eye contact, and to do so positively by default.
- Eye contact is deemed important by Western society and culture.
Dogs Can Communicate with Humans Via Eye Contact
Eye contact is important in all kinds of mammals. Especially dogs, a collection of species that evolved together with humans for at least 14000 years. There is extensive research and writing about eye contact with dogs, easily reachable by a search in your favorite search engine. For starters, although dogs use body language and other signals in many ways, they do use eye contact for a few select cases.
- Direct eye-to-eye stare – a threat, expression of dominance, or warning that an attack is about to begin.1
- Direct eye-to-eye stare to human at the dinner table, followed by direct stare at food – dog wants some food.1
- Eyes turned away to avoid direct eye contact – breaking off eye contact signals submission;1 it is also recognized as a calming signal.2
- Blinking is also recognized as a calming signal.2
(Wikipedia: Dog Communication)
1 Coren, Stanley (2012). How To Speak Dog. New York: Simon and Schuster. ISBN 9781471109416
2 Rugaas, Turid (2006). On talking terms with dogs : calming signals (2nd ed.). Wenatchee, Wash.: Dogwise Pub. ISBN 1929242360
In addition, dog breeds that are more closely related to wolves don't seek eye contact with humans as much, demonstrating that the importance of eye contact to humans also influenced the dogs we lived with. See American Kennel Club: Visual Communication – Different Breeds Seek Eye Contact Differently (summary article) and PLOS One: Dog Breed Differences in Visual Communication with Humans (research paper).
People have even written articles that draw similarities between eye contact with dogs and eye contact with other humans. For example:
A dominant dog may feel challenged by direct stare and a submissive dog can be intimidated with the eyes. ... People who are shy or intimidated by someone direct their eyes away from a more dominant personality.
(CANIDAE: What Eye Contact Means to a Dog; emphasis mine)
Other Mammals Establish Dominance with Eye Contact
Eye contact often indicates aggression with other species.
Wikipedia has some more information and references here: Eye Contact - Between Species.
Babies Respond to Eye Contact Very Early
Babies respond to eye contact early. So early, in fact, that it's one of the first major developmental milestones (at least, according to the CDC).
Parents typically notice the first direct eye contact from their baby at around 6 to 8 weeks of age. ... Eye contact indicates that your baby's neurological development is progressing normally. A baby who makes eye contact is showing that she knows what a face is and understands that facial expressions can indicate how a person is feeling.
(LiveStrong: When Do Infants Make Eye Contact?)
Eye contact can even induce a marked shift in brain patterns for both the baby and the person looking at the baby, as reported by Time: Making Eye Contact With a Baby Changes Both Your Brain Waves, Study Says.
Lack of Eye Contact can Indicate Autism
To paint with a very broad brush, autism is largely defined by the lack of typical social interaction, which includes regular eye contact. Recent research is showing that this can be identified even in babies.
The researchers tested each infant 10 times between 2 and 24 months. Initial levels of eye contact for both groups were similar. But infants who did not develop autism increased their eye contact over time, while eye contact steadily declined among the 13 infants later diagnosed with autism.
Autism Speaks: Eye Contact Declines Early in Babies Who Later Develop Autism
Social engagement skills are intact after birth in children with autism and the decline of these skills is often one of the things that parents notice as a potential marker for autism.3
MendAbility: Four simple tests for early warning signs of autism: eye contact and tracking
3 Jones, W., & Klin, A. (2013). Attention to eyes is present but in decline in 2-6-month-old infants later diagnosed with autism. Nature, 504(7480), 427–431.
Oxytocin Can Increase Eye Contact!
The scientists then tracked the eye movements of the study subjects and found that, compared with controls, those who received oxytocin via nasal spray showed increases in the number of fixations—pauses of about 300 milliseconds—on the eye region of an interviewer’s face and in the fraction of time spent looking at this region during a brief interview (Translational Psychiatry, doi:10.1038/tp.2014.146, 2015).
The Scientist: The Hormones and Brain Regions Behind Eye Contact
Note: I saw multiple claims that eye contact increased oxytocin, but did not find an authoritative source to quote here. It may be the case that this claim is derived from circumstantial evidence, such as the association of feelings of love/etc with other actions that are known to increase oxytocin.
The eyes and the rest of the face are an important source of social cues, so the lack of proper attention can mean a disconnect in proper social interaction.
“The ability of oxytocin to get the men to look at the eye region of the face more is probably very important because we receive so many social cues from the eye region,” [James Rilling] says. “If you’re not attending to those social cues, you miss a lot, and then in turn you miss the opportunity to learn a lot about appropriate social behavior.”
The Scientist: The Hormones and Brain Regions Behind Eye Contact
In addition, Western culture in particular places significant importance on eye contact.
Keep in mind there are cultural differences. Westerners prefer stronger eye contact than many in East Asian cultures. In fact, research shows that in East Asian countries, cultural norms discourage direct eye contact.
(Forbes: Learning The Benefits Of Eye Contact From A Dog)
In many cultures, such as in East Asia and Nigeria,19 it is respectful not to look the dominant person in the eye, but in Western culture this can be interpreted as being "shifty-eyed", and the person judged badly because "he wouldn't look me in the eye"; references such as "shifty-eyed" can refer to suspicions regarding an individual's unrevealed intentions or thoughts.20
(Wikipedia: Eye Contact - Cultural Differences)
19 Galanti, Geri-Ann (2004). Caring for patients from different cultures. University of Pennsylvania Press. p. 34. ISBN 978-0-8122-1857-2.
20 Kathane, Raj (June 2004). "BMJ Careers: Adapting to British culture". BMJ Careers. 328 (7454): 273.
Eye contact became important for biological reasons at first, as selected for by evolution. The eyes became one means of implicit and explicit communication, especially with regards to relationships between two different beings. Subsequent social norms then built upon these low-level biological and mammalian patterns and Western culture in particular chose to prioritize and reward more eye contact. Western culture likes steady, confident eye contact, so failure to maintain acceptable eye contact can be interpreted to mean that the other person either has something wrong with them or their motives are suspect, among other possibilities.