Unfortunately - I'm not sure. It took me two or three months to figure it out in that case, and while I remember the revelation that "This is a healthy relationship", I don't remember anything in particular sparking it.
If this relationship ends, I still don't know what criteria I'd use to identify what I'm looking for. (She messaged me on OkC, actually; perhaps the only woman to do so who wasn't threatening me or who hadn't read my profile at all, although I fixed the latter by making my profile picture deliberately off-putting to the sort of person who wouldn't read a profile in the first place.)
There's a lot of data and research on what makes people successful at online dating, but I don't know anyone who actually tried to wholeheartedly apply this to themselves. I decided to be that person: I implemented lessons from data, economics, game theory and of course rationality in my profile and strategy and OkCupid. Shockingly, it worked! I got a lot of great dates, learned a ton and found the love of my life. I didn't expect dating to be my "rationalist win", but it happened.
Here's the first part of the story, I hope you'll find some useful tips and maybe a dollop of inspiration among all the silly jokes.
P.S.
Does anyone know who curates the "Latest on rationality blogs" toolbar? What are the requirements to be included?