My first foray into solo diving was back in 1995 (Dive #480) and initially I didn't feel too comfortable about it being used to diving with a buddy and this being hammered in by everyone from instructors to other fellow divers.
I had just returned from the UK to Jeddah, and brought some Aquatica spares back for one of my "new UW photographer friends" who happened to own a boat.
He invited me out for a dive and there was only the two of us on his boat. In an area called Tower Reef he and some friends had installed underwater moorings and it was their general practice to solo dive with nobody on the boat. I was a bit surprised at this to be honest.
He set his camera housings up on 5m lines that were lowered overboard and told me to do the same with my two Nikonos Vs.
Once we had done this he basically threw his gear overboard and jumped in after it, and then after kitting up said "see you in an hour", and descended.
There I was standing alone in a boat, beautiful flat calm day in the Red Sea and the only diver within miles was already underwater. So I kitted up and splashed descending into 30m vis around Tower Reef and did a very wary 1 hour dive keeping close to the mooring area as I didn't want to wander far away and surface far from the boat, having been used to boats pick me up from the surface and not navigating back to a specific point when boat diving.
On the second dive I was more relaxed and although I spotted my "buddy" a few times on that dive I was totally absorbed by the photography and not distracted by a buddy. It just escalated from there
although I always wrote my "buddy's" name in my logbook and he signed it, just in case the "scuba police" did a check
When PADI started the Self Reliant course I was the first to take this up in UAE doing this at Divers Down in September 2012 (Dives #1099-1101), and I probably had >200 solo dives by then. The card was only to make me sort of "legal"
Nowadays I don't usually solo dive but if my regular buddy is not with me and I'm diving with my regular dive op, it's fine with them. I always have a "pony" (S30) which is more than I need to even surface from our deepest sites (30m). 50 bar on that tank is sufficient from 30m to surface.