If it's been a really long time since I last played the main game and the story has already become pretty hazy in my mind, I'll replay it with the feeling of experiencing it all over again for the first time.
But if the game left a deep and intense impression on me,
I'll be too impatient to wait and will jump straight into the FD.
And when the FD turns out to be just as amazing as the main game, after finishing the FD,
I'll end up going back and replaying the main game again to relive the story.