The remoteness of the route makes a difference. Even if I do not see people, if I am near a fairly popular area or if it only takes a day to get back to civilization, I feel less "alone". When I am several very difficult days (over many difficult passes) from the nearest trailhead or popular trail and do not see anyone for over a week, that feels a lot more "alone". Being quite introverted, I do not mind being alone; however the safety/fear factor goes up a bit. True solo backpacking in a very remote location is inherently more dangerous than with a partner or larger group. There is no immediate backup if something goes wrong.
What I miss most when totally alone is being able to have someone to help with decisions.