Apart from the other answers, I want to add that there is a significant difference between loneliness and solitude. Loneliness is when you're alone but don't want to be. Solitude is when you're alone but perfectly content with it - you may even seek it.

As humans are social creatures, loneliness tends to arise when meaningful social interactions are consistently insufficient and you feel excluded from any relevant peer groups (family & friends mostly). This, of course, is a subjective matter.