First, I'm new to this site - I just joined a few minutes ago to begin learning the tricks of the trade - so I am certainly not an expert, but excited to get better.
As far as solo backpacking, I sort of noticed all at once that I've turned down a few group hike/overnight opportunities over the past couple months that I would not have turned down a year ago. I ended up going out anyway, but solo, which is new for me.
I noticed I planned better, paid more attention to time, weather, terrain, made situational assessments more realistically (should I really cross here?), spent more time checking my gear prior to departure, caught myself conceptualizing solutions to foreseeable problems when I might otherwise have been joking around with buddies, and just generally felt a greater sense of responsibility when making decisions and executing them. (Along with this came a greater sense of freedom.)
I believe, for me, hiking with others led to a false sense of security. Perhaps unconsciously, I knew in the back of my mind that someone's got to have extra batteries, extra water, extra fuel, etc. Someone knows where we're at and how to get out. If they can cross here, I can, too, et cetera.
I've been out solo now over the past three weekends (partly due to a new pack) & have come back feeling better than I used to when out with friends. I'm not quite sure if I can articulate exactly why, though. It's probably a combinations of things ....
For example, I slipped bouldering solo through a canyon wash just yesterday. I knocked my ribs pretty good against a rock & a few items went flying from my pack. (Matter of fact, it still hurts to sneeze - lol.) At first I was stunned, but then had to assess, re-orient, and keep moving on, knowing I had only an hour or so before nightfall. I honestly feel great about it, I'm happier because of it, or feel more alive or something. I think I will learn more about life & what I'm worth going solo.