If you haven’t realised this yet, please try to recall as much the anime you have watched as you could. I am pretty sure you have already seen this in all your life. To get to the point, ロボット[robotto] means a robot. I think you already see what I am going to say…Yes, ロボット here doesn’t even necessarily mean a real robot, well something like a machine, but pretty close to a real human being. In other words, it’s a humanoid, and you can also say 人造人間[jinzou ningen].
It seems like most anime (or whatever) where a humanoid shows as a lead could be such a heartbreaking story that fans get touched. Normally, humanoids are heartless and emotionless. Humanoids are supposed to just follow the master, obey the order, and work as they are told. However as they spend time with human beings, they would get humanized. You know what I mean? It’s like a heart is planted inside him/her. They eventually display their feelings so that they wouldn’t even fulfill the mission anymore. But they know their places, so they can’t live as a human being…what shall I do?? Why am I doing this? I have to complete my mission, but what makes me hesitate? Is this…love?
OMG, this is kinda cheesy. But there is always a deep story anywhere humanoids are. No matter how close to human beings they are, robots are robots. These two creatures are not able to have a happy conclusion. The girl you love is actually such a 人造人間, but even that, if you want to believe her smile is real, you would have a ロボット萌え[robot-moe]. Speaking of such a 人造人間, I can think of Ayanami Rei in Evangelion, Nagato Yuki in the Melancholy of Suzumiya Haruhi, Chi in Chobits, No.18 in Dragon Ball, Abeno Yasuaki in Harukanaru toki no naka de, Abusolute Boyfriend[絶対彼氏], My girlfriend is a humanoid[僕の彼女はサイボーグ].
When I experienced this ロボット萌え for the first time, it was in anime film, のび太と鉄人兵団[Nobita and the Troop of Robots] in 1987. There is a girl named リルル[Lilulu], she is my first time seeing a 人造人間. I must say she is a really heartbreaker.