Why this is the most Romantic Picture in the World

Because...he's looking, eyes open, at the light. She's struggling with faith that the light will ever come, and he's not telling her that he will be her light and salvation and saying he loves her will make everything dandy.
No.
Instead, he insists that the light will come. Or, as the movie puts it "...I do not believe this darkness will endure"....
He has a sureness.
And he looks out toward the light, reminding her, the light will come.