In Roland Mueser's "Long-Distance Hiking", he documents three fundamental motives of long-distance hikers. He notes that hikers typically identify one of these as their single, main reason for hiking:
Challenge and Adventure: This is the thrill of experiencing something totally new, of doing something totally different, and perhaps being noticed and admired for the achievement.
Love of the Outdoors: These people belong to a select group of career hikers who would like to spend 12 months a year on the trail. They truly love the trail and the wilderness. If we attempt to classify any motive as a pure one, this would seem to be it.
Escape and Simplicity: These hikers are not so much attracted to the challenge or beauty of the trail as they are repelled by what life is offering them at home. These backpackers seek a change in their environment and often stress simplifying their lives.
I'm not sure what this forum poll will show, but the author summarized his survey responses as:
Challenge and Adventure- 60%
Love of the Outdoors- 20%
Escape and Simplicity- 20%
The book also goes into more depth on this topic. I found the book to be very interesting.