Why would person be fine then drop dead ?

About dropping dead: A typical cause is a heart arrhythmia or a brain attack (stroke). Otherwise, teens and young adults die suddenly in several ways. Teach teens to always think ahead wisely. That requires good, balanced parenting. Young people die by sudden impact, as in car crashes, falling from high places, etc. There is also sudden impact with a bullet. Teens or young adults added to guns are a bad combination.