(a) I don't believe I have a purpose in life. At least one that's definable as being distinct from goals/values. (Edited to add: I'm fine with that, and to an extent even happy about that). When did I realize that - sort of always, but more concrete when I see other people need a purpose in life, and noticed that I didn't seek one.
(b) Passion - too many to list, but briefly, history, politics, art, civilizations, and more recently, attributable to the amazing night skies we are having around here - backyard astronomy. Activities that involve my passions include reading, watching documentaries, visiting other cities and countries to learn, and recently, tinkering around with my telescope. Since age five or so, been in awe of night skies.