Avia,
I think you hit on two important aspects of an unreachable goal. First of all, it may just be a dream that we won't be able to achieve. Dreams are important, but we all have those "dreams" that we know aren't reality. I dream of retiring at 40 to a beautiful ranch in the mountains...Not happening!
The other thing is that a goal may be reachable, but if the person has not desire to achieve that goal (for whatever reason) it essentially becomes unreachable. We cannot want something more for someone than they want for themselves.
Ryan