I believe I discovered mine when I listened to one of Steve's podcasts.
StevePavlina.com Podcast #015 - What Is Your Purpose?
It was quite awhile ago now, so I think I shall listen to it again.
In any case, what I came up with still resonates with me after well over a year.
For what its worth, I believe I was put on the earth to spread beauty to others. I also feel a duty to work to conserve the feline populations of the planet; I've always been really drawn to cats my entire life, and it pains me to see what human kind has done to most of the large cat populations. I feel these two things actually dovetail each other quite nicely, as one of the best ways to conserve these creatures besides breeding programs is to educate others and share the beauty that these kitties possess.
I also feel drawn to spreading beauty in the human form, and helping to shift our society's views on the human body.
Good luck on your journey!