Extract from how its there on wikipedia.
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"
Loneliness is a complex and usually unpleasant emotional
response to isolation. Loneliness typically includes anxious feelings about a
lack of connectedness or communality with other beings, both in the present and
extending into the future. As such, loneliness can be felt even when surrounded
by other people. The causes of loneliness are varied and include social,
mental, emotional, and spiritual factors.
One can experience
loneliness for many reasons and many life events may cause it, like the lack of
friendship relations during childhood and adolescence, or the physical absence
of meaningful people around a person."