I read something on a fantasy website sometime ago and found information holding some truth that i think to share with others. Before you begin to judge, just remember that true knowledge can sometimes be found in the most unlikely of places.
I believe it's common knowledge by now that anyone can become an energy user otherwise what would be the reason for this website. All human beings have that spark within us that allows us to utilize the raw power around and within us. The Art of using this ability is commonly known as Magic or Magick(More correct spelling in my opinion).
A mage, sorcerer, witch etc learns to harness this power to change their reality to whatever they desire though usually this is something small. By this logic someone who has learnt to use magick should naturally be able to learn to,say, fly without any means of aircraft or trickery right? Alas, this does not seem to be the case. Most of the people here say that it is because a human being cannot bend, change, or manipulate the laws of nature.
The theory I have is that the main reason why such a thing is not possible is not because of the 'laws' of nature, rather it's because people do not believe that such a thing is possible. There are about 7 billion people on this Earth and counting you know.
As the majority of this number do not believe it is possible, it has thus been rendered almost impossible via the collective consciousness. It is that shared belief that shapes most of our reality and forces the fixedness of the laws, causing the improbability of such an occurrence to be very high.
This makes it that only powerful mages are actually able to cause such things like flight,transformation etc to actually occur, that or summon a really powerful spirit to cause it to happen. Most energy users nowadays work by 'going with the flow' so to speak. Rather than directly causing a desired effect, they influence the energies around them to create circumstances to bring about the desired effect e.g Example 1: Someone casts a love spell and Bam! the next day the target is in love with the caster, instantly. Example 2: Someone casts a love spell, after a certain amount of time the person breaks up with whomever they were dating because they found them cheating, notice the caster in a moment of vulnerability, falls 'naturally' in love with him/her, yada yada yada.
In Essence: the belief of human beings as a whole race is not something to mess with and would literally be equated to something akin to a powerful God(ess).
This is my theory as to why certain magicks cannot work and yeah, it may not be completely original but there is a chance that it could be true. Take it as you desire.