Spoilers below

• The obvious tragedy in One Day is when Emma gets hit by a car and dies just before her and Dexter symbolically complete their reunification with their wedding.
• The greater tragedy is actually the fight they had at the fancy restaurant where Dexter was unable to empathise with Emma and alienated her causing them to drift apart.
• There was nothing either of them could have really done about the car but Dexter had all the agency in the world to put his ego to the side. Real tragedy comes from the wrong choices actively taken rather than random events that may befall us.