As if our brains magically emerged from a billion years of evolution with complex mechanisms that did not help us survive... :)
I tried (like a good nerd) to reverse engineer all our emotions in my book "The Psychopath Code." Loneliness fell into my group of "Tribal Emotions" (thus evolved to help us form extended families), and here is how I defined it:
"Loneliness - the emotion of being disconnected. You feel physical pain in your intestines and chest when you see others enjoying themselves. You sleep lightly and wake often. Your immune system starts to slow and you start to get sick. Your metabolism slows down, yet you eat more often than usual. Your circulation slows, and you feel cold."