Some things were not meant to be easy. This is a lesson that comes hard to some and harder to a few, and for Roy Mustang, it was a very hard lesson indeed. He went into the Ishval War cocky and left broken, and came out of his room, guided by Maes Hughes, two weeks later as something different.
Mustang is possessed, maybe, but not with the feverish intensity that will someday be the trademark of the Full Metal Alchemist. His is something more peculiar to himself, a sort of thick determination that depresses other needs and desires and enables him to swallow his pride and bow in front of Major General Hakuro. It allows him to submit to the unjust. It allows him to hope, and want, and obtain, rank after rank, until he stops the pain that he hates.
Someday in the near future Edward Elric will become the Full Metal Alchemist, and Mustang will see in him the kind of want he has, but different, more hurt and more immature. A part of him will admire that fire and free will ... part of him will wish that he had that in himself. And then he will reach out to guide that fire into his own, to protect it and shelter it and fan it to burning intensity. Mustang will want Edward Elric to succeed as much as he wants himself to succeed, and someday, that desire may burn him.
But in the crucible of life, there are people that are catalysts, who never seem to change, and for Mustang, that person is Maes Hughes. Would Mustang have emerged from his room at all without him? What withered and dead element would he be? Hughes has always been there and always will be, friend, supporter, co-conspirator.
But someday, in the future, that catalyst will die. And between the fire of Edward Elric and the loss of that dear friend, Mustang may lose sight of his goal.
The pain he hates might be too close, then.