How about you, what tipped the balance to the dark side?
A good question, and one that prompted a lot of thought - I really hadn't considered it before.
As a caveat, I thoroughly enjoy buddy diving, I thoroughly enjoy group diving, I thoroughly enjoy solo diving. So, I haven't established solo diving as my primary style, or even as a preference. It is just something that I do when I want to, and which I very much enjoy. I have honestly gotten to the point where I don't think of it as conspicuously riskier, or even daring. Maybe I should. I guess an argument could even be made for incipient complacency. But, I do not think that's the case.
If there was a defining moment, when i realized that I really (really, really) enjoyed the freedom of diving alone, it came on a night shore dive on Bonaire almost a decade ago. A group of friends (7-8) were doing a night dive. There was no particular leader, or even identifiable buddy pairs / trios. It really was just a group of fairly experienced divers swimming along, more or less together, enjoying the wonders of the reef at night. I was swimming somewhat ahead of the group, and gradually noticed that the lights behind me were appearing less intense. I turned around and realized that most of the group had apparently seen something really cool on the reef, and were clustered around an area shining their lights on 'something'.
Rather than swim back, I decided to continue on ahead of them figuring we would probably reconnect when I turned my dive. So, now I was diving solo, and after a few minutes, on a whim, decided to turn my light off. Wow! There I was, all by myself, with no 'local' light, yet I could 'see' lots of things on the reef. There was some light for the moon (I was at about 50 feet) I truly felt relaxed (incredibly relaxed), and absolutely free - I was floating in warm water with a universe of life around me. And, that was the moment when I realized how much I liked being in the water by myself.
If that was 'the' moment, there have also been a continuing series of dives where my comfort with solo diving has been reinforced and enhanced. And, from my perspective - and this is something I tell student divers (e.g. Self-Reliant Diver students) - everyone who dives will ideally have similar experiences. Every time, you challenge yourself, with proper preparation and planning, to expand your diving 'envelope', you come away a better diver. You have greater insight into your skills (what you do well AND what you need to improve), and the unknown becomes a little less daunting. So, when I made my first solo ocean boat dive, my first solo decompression dive, I learned from each experience, I gained more confidence, I improved.