The Social Care Institute for Excellence (SCIE) says "social isolation and loneliness have a detrimental effect on health and well-being". 

Some consider loneliness a primary emotion, on a par with fear and anger. For millions of years, survival relied on being part of a gathering; being isolated from your community was dangerous. 

But now, a fear of loneliness can keep us in a toxic relationships, unfulfilling jobs, or destructive marriages.