Primates need to belong to an intimate social group in order to survive; this is especially true for humans.
Separation from your group (either finding yourself alone or finding yourself among a group of people who do not know and understand you) triggers a fight-or-flight response.

Our bodies perceive being alone (or with strangers) as an emergency. Thus, our nervous system evolved to produce the anxiety we associate with loneliness.
We breathe fast, our heart races, our blood pressure rises, we don’t sleep. We act fearful, defensive, and self-involved, all of which drive away people who might actually want to help, and tend to stop lonely people from doing what would benefit them most: reaching out to others.

According to historian Fay Bound Alberti, modern loneliness is the child of capitalism and secularism.
Many of the divisions and hierarchies that have developed since the 18th century (between the individual and the world, individual and community, public and private) have been naturalized through the politics and philosophy of individualism.

• Before the 20thcentury, about 5% of all households (about 1% of the world population) consisted of just one person. That figure began rising around 1910, driven by urbanization, the decline of live-in servants, a declining birth rate, and the replacement of the traditional, multigenerational family with the nuclear family.
• During the past half century, for the first time in human history, great numbers of people (of all ages, in all places, of every political preference) have begun settling down as singletons.

... is that lonely people can’t see that lots of people feel the same way they do.
Loneliness seems to be such a painful, frightening experience that people will do anything to avoid it. We shut down the lonely, afraid that this might be some kind of a contagious situation.