why can't Jonas and the Giver simply" apply for a change of rules" to make their lives easier?
By definition, the role of the person selected to be the Receiver of Memory is not intended to be an easy one. The community has been designed and structured to allow all its residents a safe, uniformly productive, stress-free existence. The expectations and regulations governing their lives enforce conformity with the norms of the community, which is perceived as being beneficial for all.
However, the Council of Elders recognized as the community structure was being developed that there was Elsewhere and that there had been an uncontrolled past, and that the community might somehow, someday be impacted by these external forces. The Receiver of Memory was designated as the one person in the community who would be aware of all that had been or could be experienced in those other places, since other community members were unable to imagine or understand those events or emotions. Recognizing that this responsibility would entail sensations "of a magnitude that none of us here can comprehend because it is beyond our experience," the Receiver of Memory, who becomes the Giver when Jonas is named as the new Receiver of Memory, must accept and endure all the pain and anguish that is part of being the keeper of the past.
If Jonas and the Giver changed the rules controlling their unique position in the community, they would be destroying the structure of the entire society.