#lain is like

This is going to be a super bummer answer but honestly discipline and punishment is what makes him feel nostalgia. The King would often package up discipline and punishment with "I'm doing this because I love you" and after a while it sorta warped Lain's whole perception of like love and such. If someone is correcting his mistakes or punishing him for disobeying it means that they love him. Which is also probably why he does feel positively towards that feeling. It makes him think about how much the king loved him.