https://www.scientificamerican.com/article/the-loneliness-of-the-social-distancer-triggers-brain-cravings-akin-to-hunger/

Loneliness hurts. It is psychologically distressing and so physically unhealthy that being lonely increases the likelihood of an earlier death by 26 percent. But the feeling may serve a purpose. Psychologists theorize it hurts so much because, like hunger and thirst, loneliness acts as a biological alarm bell. The ache of it drives us to seek out social connection just as hunger pangs urge us to eat. The idea is intuitively satisfying, yet it has long proved difficult to test in humans.

On March 26, however, just as the COVID-19 pandemic gripped the world, researchers from the Massachusetts Institute of Technology posted a preliminary report on bioRxiv. It is the first study in humans to show that both loneliness and hunger share signals deep in a part of the brain that governs very basic impulses for reward and motivation. The findings point to one telling conclusion: our need to connect is apparently as fundamental as our need to eat.