Themes
Then never less alone than when alone.
— Samuel Rogers · Human Life (1819)

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The line paradoxically claims that being alone is when a person feels least solitary. Solitude is presented as full rather than empty, perhaps because thought, memory or reflection supplies company. The wording is calm and consoling, treating time alone as a kind of companionship.
Context reviewed