I don't know why I got so invested in this story exactly, but maybe the actors are just that good.
I actually cried in the last few episodes, and I am not someone who will cry for anything (maybe tear up, but with this I was sobbing).

The plot is somewhat simple when it comes to romance stories, but there is the twist that the ML is actually a robot who starts to develop human emotions. I went into it thinking it would be cute and funny - which it was - but I ended up staying because even the moments with the side characters were endearing and funny.

The shining star is of course Yeo Jin Goo as the robot Zero Nine, who is designed to give love, but starts to want to receive love and feel real feelings of longing and jealousy as well. However, the development of these emotions might actually cause his own destruction as a robot is not equipped to have these. I just couldn't help but want him to be happy and loved the whole way through.

I feel like the FL gets bashed a lot on the comments about this show, because at first she is a little mean to the robot ML, but I understand her point of view - she was hurting from a previous break up of a long term relationship and she felt that love wasn't a real thing that could last. Slowly she starts to grow and warm up to the ML and it's lovely to see it happen.

As I said before, the side characters were also really interesting and I think the actors did such a good job. Even though I loved the ML so much I couldn't help but second lead syndrome for Hong Jong Hyun. Also, Hong Seo Young is really good as the villain, because she is absolutely hateful (the villain Diana is up there with Dolores Umbridge on levels of hatefulness in my opinion).

The only thing I was a little disappointed in was the ending, as I which it had been more clear either way (happy or sad or bittersweet), but it didn't take away from all the enjoyment I got from watching the show.