Well for one simple reason: When you "know you could never be
   something", you ALSO know that you could be wrong... and find
   out that you CAN be something. Also, since the "know you could
   never be something" LIKELY came from a source that's NOT you,
   you know THAT person can be wrong and there's most definitely a
   satisfaction in proving somebody wrong, especially when they're
   casting you in the incorrect mold.