III,4 brings the turning point in the drama. Ferdinand proposes to flee with Luise, but she refuses. In an instant, his love turns into suspicion and rage. To understand his shock we'll return to the end of act II.
The interview with Lady Milford has seriously undermined Ferdinand's composure. He was determined to despise the sovereign' concubine, whom his father would force him to marry, and ready to insult her so that she herself would cancel the plans. Instead, he finds a victim of circumstances, an impoverished exile rescued by the prince, whose lover she became on condition that his people henceforth be governed in a humane and compassionate manner. An English aristocrat of true nobility, dignity and integrity. Ferdinand is clearly taken in.
It takes a breathless visit to Luise's house and an equally hasty confession, and Luise's instant resigned withdrawal, that threngthen his resolve to reject his father's plans. He commits himself more and more strongly as his father enters and procedes to insult everyone in the Miller household, Luise in particular. The break between father and son appears complete when Ferdinand threatens to expose the president of whose corruption he is all too aware. He has now risked all for his love.
And now Luise will not flee with him. Not at the cost of incurring his father's wrath. His curse will follow them everywhere, she says. She'd rather  sacrifice her own happiness and return a lost son and defiant  one to his father. She is willing to renounce a relationship that would defy established order, hallowed by tradition and divinely sanctioned. What is meant of course is the separation of classes which for a moment seemed conquered by their love. But only for a moment. The world in which she has grown up, miserably narrow and stifling, allowing little room to breathe and no room at all to move, has a powerful hold on her. The status quo is God's will, she was a criminal wanting an exception. She is convinced of it. Now she is willing to accept her loss of happiness as punishment.
Suddenly the difference between them is no longer one of class alone but of education as well. This dreadful dependency on beliefs that have informed her all her life comes as a total suprise to Ferdinand who has been away at school where he heard a different doctrine. Willing to sacrifice all he is incapable of understanding why she can't. He'd rather assume that it is a secret lover that holds her here. Incongruous? Yes and no. Yes, for all the evidence he has had to the contrary. No, because he has known corruption all his life, corruption is the norm, religion teaches it, experience shows it. Luise seemed to be the exception, it was too good to be true, he couldn't  quite believe it, and now he doesn't.