Before the crash, Jin was consumed by his work for Sun's father, and Sun was planning to leave him. The Island strips away the power dynamics and cultural pressures, forcing them to communicate honestly -- complicated by Sun speaking English and hiding it from Jin. Their separation across time (Sun in the present, Jin stuck in the 1970s) is agonizing. Their reunion in Season 6 is brief and beautiful. They die together in the submarine, holding hands -- choosing each other one final time.

1. S1E6
   House of the Rising Sun

   Flashback reveals the fractures in their marriage. Jin handcuffs himself to the wreckage.

2. S1E17
   ...In Translation

   The truth about Jin's work for Mr. Paik. Sun reveals she speaks English.

3. S6E10
   The Last Recruit

   Sun and Jin reunite after years apart. Brief. Joyful.

4. S6E11
   The Candidate

   They die together in the submarine. "I won't leave you."

Pre-crash: marriage in crisis. Season 1: forced to confront their problems without escape. Season 2-3: rebuilding trust. Season 4: separated (Sun leaves the Island, Jin stays). Season 5: Jin in the 1970s, Sun searching for him. Season 6: reunion, then death together.