Everything happened so fast. Lex had always thought that his separation from the two would be something he'd see a long way coming, for example the end of their deadline. Although Lex had done his best to remain detached, and had not actively helped them out during their struggles, he had to admit that he was still somewhat attached.
The two of them were idiots, basically. Despite living such long lives, overcoming such impossible odds, and achieving such great things, considering where they started out from, they were still just kids who had a bit of strength now.
Sure, life had forced some wisdom down their throats due to all their experiences, but for the most part they were just two kids who were hopelessly in love with each other. If Lex hadn't grown immune to the cringe one liners, and their mad displays of affection for one another, he might have gone crazy.
