For primitive peoples, who relied upon fire for cooking and warmth, catching on fire was a known dilemma, and the pain from such an accident not unknown. Thus, as one of the most excruciating pains, and one slow to heal and suffering the injured much pain during the healing process, eternal fire was the worst of the worst. Similarly, the higher one went, physically speaking, the better life got. Specifically we are describing living on hill tops versus valleys, and having the higher campground versus one downhill. As the saying goes, shit flows downhill.
Valleys have dampness and humidity, lizards and snakes and insects galore.
Hilltops have breezes and vistas. Better yet is what the birds enjoy, high in the skies. Thus, heaven is viewed as up in the skies, and the higher the better.