My Best Friend's Wedding(1997)

A 28-year-old woman, who years earlier made a pact with her closest male friend that if neither were married in ten years time they would marry one another, now faces the fact that the man is about to marry someone else. He asks her to be his "best man" and she agrees, planning to go to the ceremony and then break up the couple before they exchange their vows. Instead, she accepts the turn of events and realizes she doesn't need to be married to be happy.