First off, I am going to establish that I sometimes dive solo. I am a scuba instructor for an agency that promotes buddy diving.
I will not hike in the wilderness alone. I will not ski the back country alone. In general, I will not dive alone. Now, I do practice in the pool alone, and I will briefly set up / clean up instructional materials in shallow sites alone, but that's about it.
Those are my decisions.
What are yours?
Bob and John are two of my favorite SB Instructor posters; IMHO, each of us has a significant level of training and experience, but we live in different worlds.
I grew up in Colorado / the Rockies; raised by a serious Wolf Creek powder hound who's last technical rock climb was a near death experience on Ship Rock back when it was legal to climb (they did summit - he did not lead). Dad also taught me to scuba at age 7, and my most memorable dive that summer was the St. Joe river in Northern Idaho on one of our many white water canoe trips.
Since his death when I was 25, I have found very few adventurers like dad, so he is still my #1 buddy, which causes everyone else to see me as going solo. One of the reason I do not take "living" buddies on my most serious adventures is because I do not want to be the cause of someone else's injuries/deaths.
Bringing a couple badly injured best friends out of the wilderness in my twenties, one of them still addled from that head injury now 30 years later, and numerous near-buddy-death non-injury events have led me to really prefer solo when I'm on the edge.
John lives far away from every day dive sites, Bob lives in challenging dive country, I live in warm water paradise. John really does not solo, Bob solo's some, I solo a lot; location, location, location. And I am not just talking about where we live today; it is also about how we grew up, IMHO.
I do not think John would be more at risk if he were to solo dive, but he is comfortable with his decisions. I think Bob could easily do more adventurous solo dives, but I am comfortable with his decisions. I have plans to do more adventurous solo dives and my step mom and step dad are resigned to my decisions.
One of the reasons I live in Hawaii is that I am prone to causing avalanches in the Rocky Mountain backcountry. My last spring living in Colorado I made 12 solo backcountry snowboard runs; caused 5 small sluffs (rode one), 2 medium powder slides (rode one) and out ran a very slow slab slide.
Compared to my solo free climbing, white water and snowboarding in the Rockies, me solo diving (including breath hold) is what my family prefers.
PS - I also rode my '84.5 Willie G Special (the "parts bin" special) solo; from Denver to Sturgis, for a week around Sturgis and back to Denver; possibly the riskiest week of my life!