Sorry it's a bit long, but the "first 100 dives" got me thinking/reminiscing...
I learned during OW class that not everyone is immediately comfortable underwater. I had already planned a trip for me and girlfriend to dive old mines (open) in Minnesota. During OW we met a couple and talked about them coming with us. The instructor pulled me aside and recommended I don't do that as they were going to get certified but needed more structure for their first dives. By the end of class (after now paying attention), I agreed with him and enjoyed my dives with no worries.
I found I like any/all diving - from ice diving (wet suit!) to tropical waters.
I learned even smart experienced people can be very stupid/over confident. On my first trip I got to see a guy be very disrespectful to the dive operator and do a bunch of stuff I was just taught not to do. (someone who actually held some diving records and was a high IQ individual) We saw him get seriously bent. Saw the dive operation do what he said and not what they should have - took 24 hours to get him off the island and to real medical care. I'm not sure how he ended up but heard he was wheelchair bound months after trip. It made what the instructors said very real - paying attention to details, times, depths, etc. is always forefront on every dive.
I learned buddies sometimes don't tell you how uncomfortable they are. Had to stop a dive, grab my buddy by the bc and hold them still to calm them down when the night dive and surge got them near panic. Be honest when going into and during a dive!!
I learned fixing gear on the fly is doable. Spent 4 hours on Christmas eve in 0 degree weather out on the edge of the ice re-warming frozen gear, swapping regs, and adjusting gear for a 17 minute dive in a goose crap bottom covered quary - with DPV's! Still consider that a good dive - a buddy as crazy or more for diving was important for that one!
I learned my buddies - there were 3 of us on this dive - felt confident in me (and I was too after the dive) to leave me behind when on a night dive (different one). I stopped to film an octopus, turned around when I was done to find total darkness, not a light in sight! Although I did not surface after a minute (per training - but I would have been a long way from the boat), I felt confident finishing the dive - I assumed upon my return (along a wall) that the boat would still be there. After several minutes I saw flashlights in the distance, rejoined the group and had a great (and basically first solo) dive. Turns out they went over the wall out to open water to play with the bio-luminescence and said they felt I would be fine - which it turns out I was.
I have also learned that a solo dive is not a four letter word. ok well it is, both of them, but the point is with experience,
proper gear, and useful knowledge it can be a very relaxing and enjoyable endevor when a buddy is not available.
I've also learned you CAN fall asleep while underwater - sometimes safety stops are a bit too relaxing. I'm not sure I actually fell asleep, but pretty sure there were a few seconds of zzzz - more than once actually...
I've continued to learn on every dive I've been on. Skills, people, gear, etc. Advanced classes are good - diving after them to practice the skills is what makes them useful.
Lastly I learned that wanting to dive all my life was a correct feeling - from as young as I remember to now
(diving for 25yrs). I prefer warm water reefs but doubt I would turn down any dive I could make. Dive on!