Although a "hug" does not require closed eyes, doing so is a strong sign of affection and love. It's as if your emotional brain is trying to "drink in" the entire moment. Physiologically, our visual systems take up about 40% of the mass - and blood supply - of our brains. The visual sense also makes up 90% of our sensory input. It tends to often overpower our senses of smell, touch, taste and hearing - as well as our cognition and emotions. When we close our eyes, this allows a bit more blood (and thus more oxygen) to be directed to other portions - including the emotional components - of our brains. This phenomenon is biological. While certainly not the only warning sign to watch for - parents who consistently lack eye closure during the hugging their children - also tend to lack affection and love for these same children.