According to Vice.com, loneliness is harmful because it makes a person stressed, and stress increases inflammation: - https://www.vice.com/en_asia/article/ywxypm/being-lonely-is-worse-than-smoking-15-cigarettes-a-day It also talks about how Generation Z (18 to 22) has the highest loneliness score of all the age groups. Ironically, this could be because of social media. Julianne Holt-Lunstad is the researcher often cited as the source of the 15 cigarettes a day comparison: - Holt-Lunstad, Julianne. “The Potential Public Health Relevance of Social Isolation and Loneliness: Prevalence, Epidemiology, and Risk Factors.” Public Policy & Aging Report 27, no. 4 (December 30, 2017): 127–30. https://doi.org/10.1093/ppar/prx030.