According to Osho, loneliness is the felt absence of the other — a hurtful gap born of the ego's need for 'thou.' It is sadness, dependency, a kind of illness. Your attention fixates on what's missing. True spirituality is not loneliness but aloneness: the joyful presence of being, self-sufficient, whole, where both 'I' and 'thou' dissolve.
Loneliness is when you feel empty because you want someone else; aloneness is being happily full inside without needing anyone.