To be alone is to be ‘all-one’, living in Oneness, in solitude. As such aloneness is a truly beautiful situation, which can turn sour if there is any longing for the situation to be different, when aloneness becomes loneliness, a mental disturbance.
Old English eall ‘all’, from PIE-base *al- ‘all’ and ān ‘one’, from PIE base *oi-no-.