Passion is a fire that is in your heart and that will forever try to get you do something you must do. Passion makes you alive—otherwise you'll spend decades withering away. Passion is never an idea, it's always something that you actually do. Passion is orthogonal to happiness, happiness may or may not come with or without passion.

Interestingly, From The Collaborative International Dictionary of English v.0.48 [gcide]:

  Passion \Pas"sion\, v. i.
     To suffer pain or sorrow; to experience a passion; to be
     extremely agitated. [Obs.] "Dumbly she passions, frantically
     she doteth." --Shak.
     [1913 Webster]