ELIZABETH: You'll soon feel better when you get out of here.
FRANKENSTEIN: It's like heaven being with you again.
ELIZABETH: Heaven wasn't so far away all the time, you know.
FRANKENSTEIN: I know. But I didn't realize it. My work. Those horrible days and nights. I couldn't think of anything else.
ELIZABETH: Henry, you're not to think of those things any more.
After the monster kills Fritz, Frankenstein comes to his senses. Suddenly, he's not mad at all anymore; it's like he's a completely different person. Madness is presented as a passing illness, but also as a choice. Frankenstein gave up on his fiancée, and chose to defy the natural order of things. That made him mad. But once he goes back to the fiancé, all is well again. Madness is less a problem inside Frankenstein than it is a problem with his relationship to society. All he needs to do is take off that lab coat, and his brain starts to work right again.