“Loneliness isn’t being alone, it’s feeling alone". In other words, it's not about having social contact but about feeling like you have meaningful relationships.
This feeling, it turns out, was increased or was more likely to occur when one person had contact with a person who already reported themselves to be feeling lonely.
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The spread of loneliness seems to have its own particular characteristics. Women, for example, seem to be more susceptible than men. Also, the more lonely people a person knows, the more likely she herself is to become lonely. That trait distinguishes loneliness from something like alcoholism: Having an alcoholic friend increases your odds of becoming an alcoholic, but having three alcoholic friends makes you no more likely than having just one…
Source: here