What is the meaning of contrast? The hypocrisy of reality, the twist of fate?
For Leo, it was the perverse dream of powerlessly watching how Mei disappears from his life again, over and over again.
The pain of watching her go, the struggle of losing her no matter how hard he tried. Filled with fear, his night-released mind continued to paint all sorts of different scenarios, ranging from Mei simply dissolving into a mist, through having her go down with some lethal sickness and going as far as her simply... leaving.
Stuck in those nightmares, Leo couldn't even decide what was the worst of the scenarios his terrified mind continued to produce as if all of his neurons went into overdrive and agreed to work overtime to further terrorize him.
Was it better to be powerless and watch her die? Or was it worse to stop himself from doing anything when she expressed a desire to leave before acting on it?