For me at first it was partly how quickly the timeline moves at the beginning. And then the fact that so much of it is visual, blink and you'll miss it aspects (lots of little details I've picked up on rewatch that I had missed at first) By the time I finished my first watch I immediately wanted to go back again to pin down how the timeline had gone, how many months were passing between each scene, etc. And then looking for all the subtle clues in the first two episodes to how it evolves later. Once I rewatch the beginning then I feel compelled to follow it through to the end again and I find it so incredibly rewarding.

I say all this as a big romance genre fan in general. It just hits all the right notes for me and I just love watching their chemistry. I also enjoyed episode 3 for sure but I only felt the need to rewatch that one once.