Interesting subject, i have been thinking about this subject for some months, and this is what i think, i may be wrong though so feel free to correct me
Almost all dreams are posible, the problem is resource management, if you dont have the right resource then you cant reach that specific dream, so you get the resource, or you just conclude its un-realistic.
The problem is people, more presicely their minds, hundreds of reasons for making mistakes and taking the wrong paths and attitudes, thats the main obstacle to reaching ones dreams. I agree with what Uplift says, "conditioning" defines our minds, as you know we CAN change our minds, but most people would not want to do it because of their conditioning.
The mind is the one who decides actually, not resources, if you have an un-healthy mind resource management wouldnt make a difference.
So i think it goes like this, for people to reach their dreams they must change their minds, to manage their resources succesfully, to reach their goals that accomplish their dreams.