depression

From Lost Connections, nine causes of depression and anxiety:

1. Disconnection from meaningful work
2. Disconnection from other people (loneliness)
3. Disconnection from meaningful values (consumerism)
4. Disconnection from childhood trauma
5. Disconnection from status and respect
6. Disconnection from the natural world (due to biophilia)
7. Disconnection from a hopeful or secure future
8. Genes
9. Brain changes

Depression and anxiety occur together, often resulting from social isolation

Lost Connections suggests that depression and anxiety are two sides of the same coin:

Everything that causes an increase in depression also causes an increase in anxiety, and the other way around. They rise and fall together.

and that both are often both are the understandable physiological result of loneliness or disconnection. Over the course of human evolution, this would make sense:

Now imagine if—on those savannas—you became separated from the group and were alone for a protracted period of time. It meant you were in terrible danger. You were vulnerable to predators, if you got sick nobody would be there to nurse you, and the rest of the tribe was more vulnerable without you too. You would be right to feel terrible. It was an urgent signal from your body and brain to get back to the group, any damn way you could.

Research demonstrates that social isolation causes anxiety—hypervigilance to threats—as well, which can make people less trusting and less likely to reconnect socially, a tragic accelerating feedback loop.

John [Cacioppo] discovered a cruel twist in this story. When he put lonely people into brain-scanning machines, he noticed something. They would spot potential threats within 150 milliseconds, while it took socially connected people twice as long, 300 milliseconds, to notice the same threat. What was happening? Protracted loneliness causes you to shut down socially, and to be more suspicious of any social contact, he found. You become hypervigilant. You start to be more likely to take offense where none was intended, and to be afraid of strangers. You start to be afraid of the very thing you need most. John calls this a “snowball” effect, as disconnection spirals into more disconnection.