A lot of the comments seem to be discussing social anxiety, which is not the same as loneliness. In my late-teens and early-20s I was extremely anxious in social situations but I did not feel lonely. Social anxiety, and the isolation that often results from it, can contribute to a person feeling lonely but the two concepts are not synonymous.
As I understand it, loneliness is a psychological state that results when a person feels deprived of meaningful contact with other humans. Very few people can maintain their sanity and well-being completely cut off from their fellow humans (the proverbial hermit living contently in the wilderness with only himself for company is a rare phenomenon) but how much and what type of social contact an individual needs to maintain their equilibrium varies wildly.
So, it is impossible to determine how lonely a given person is only by looking how much time they spend alone or how anxious they are in social situations.
As I understand it, loneliness is a psychological state that results when a person feels deprived of meaningful contact with other humans. Very few people can maintain their sanity and well-being completely cut off from their fellow humans (the proverbial hermit living contently in the wilderness with only himself for company is a rare phenomenon) but how much and what type of social contact an individual needs to maintain their equilibrium varies wildly.
So, it is impossible to determine how lonely a given person is only by looking how much time they spend alone or how anxious they are in social situations.