One character (or both) must make a conscious, difficult choice. They choose vulnerability over safety. They choose the relationship over their old identity. The grand gesture isn't about a lavish gift; it's about proof of change. The rigid detective breaks a rule to save the smuggler. The cynic finally says "I need you" without irony.
This is the gold standard. The slow burn relies on proximity and denial. Think Darcy and Elizabeth. Every interaction is laced with subtext.
Every great romantic storyline has a "dark night of the soul." This is the misunderstanding, the betrayal, or the obstacle that forces the couple apart. However, the modern reader has evolved. We no longer accept the "Idiot Plot" (where a 30-second conversation would solve everything). The best ruptures come from character flaws, not miscommunication.
Think of Normal People by Sally Rooney. The rupture isn't a villain or a lie; it is the protagonists' own inability to articulate their worth. That is compelling tragedy.