-Why do I live?
-Ask God.
When he is told to imagine a world in which Maria no longer exists, he sheds a tear, and one of his eyes gains colour. In other words, Mon gained part of his humanity back. Before meeting Higumadon, he was a demon, but after learning about the limits of his strength when he was defeated and killed in that hallucination, he gained the ability to "imagine"/empathise with the pain of others. But just empathy isn't enough to turn a demon into a human. By connecting with Maria, Mon 'returned' to his childlike, innocent self before his childhood trauma morphed his mind. Slowly, the ability to have morals and value people began manifesting, and this scene is the perfect showcase of his development. After re-reading I can see what the author intended with it
