Quick answer: The theme of Asimov’s story is that the misuse of technology and the abuse of the planet’s natural environment would inevitably lead to man’s demise.
At the time Isaac Asimov penned his short story "Hallucination," the country was just coming out of an “energy crisis” and the Cold War was passing through one of its zeniths. Global tensions were running high, and environmental concerns regarding Reagan Administration polices were at a peak. Asimov’s story about a 15-year-old boy sent to Energy Planet to serve a three-year tour with a military unit responsible for facilitating the exploitation of other worlds to feed Earth’s seemingly inexhaustible thirst for energy could be considered an allegory about the need for sustainable development and more responsible approaches to man’s stewardship of his environment. It is also a warning against the misapplication of the advanced technology that seemed a preoccupation of the then-administration.
Sam Chase, despite his superiors’ avoidance of the subject of hallucinations allegedly experienced by other inhabitants of this planet, is determined to explore their existence and meaning. Upset about his assignment, his immediate supervisor, Donald Gentry, admonishes his newly-arrived conscript about questioning the decisions of the machines upon which humans have come to depend: “Surely the Central Computer, which evaluated your scholastic record and your social and personal background can be trusted in its judgments.” Sam, however, is naturally inquisitive and, at the first opportunity, explores the planet outside of the man-made dome in which the humans live. The search for sources of infinite energy (“It was the magnetic field that would be tapped. Energy would be led away in enormous amounts and yet it would all be a pinprick, less than a pinprick, to the star’s rotational energy, which was the ultimate source. It would take billions of years to bleed off all that energy, and in that time, dozens of populated planets, fed the energy through hyperspace, would have all they needed for an indefinite time”) may dominate man’s purpose with regard to his expanding reach throughout the known universe, but Sam becomes convinced that this alien world has a different agenda.
On his explorations, Sam discovers that the phenomenon that was interpreted as hallucinating was, in fact, the planet’s natural environment warning its human interlopers against manipulating the natural order, and that the “hallucinations” were the planet’s means of communicating with humans. Asimov’s story is intended as an indictment against man’s proclivity for expanding his reach and dominating his environment to the ultimate detriment of all. The plants and insects native to this planet will be destroyed, as humans have systematically destroyed their own native world. As his narrative continues, the author suggests the inevitability of such developments:
Human beings had built the Dome, cleared a large area of all planetary life and substituted their own. And once the neutron star had its power station—once floods of energy moved outward through hyperspace to power-thirsty worlds—more power stations would be built and still more. Then what would happen to Home... This planet was the nearest convenient base to the neutron star. It would be flooded with more and more people, more and more Domes, and their Home would be destroyed. "But you could change our minds if you had to, even if you damaged a few, couldn’t you?" If they tried, people would find them dangerous. People would work out what was happening. Ships would approach, and from a distance, use weapons to destroy the life on Home, and then bring in People-life instead. This could be seen in the people’s minds. People had a violent history; they would stop at nothing.
The military and political intrigues characteristic of the Cold War, during which much of the developing world had devolved into a chess board upon which the two superpowers maneuvered, was wreaking death and destruction across the world, and the competition and search for energy was destroying what warfare hadn’t. As Asimov’s protagonist, the innocent, inquisitive Sam, notes at one point regarding the mysterious force responsible for the hallucinations, “It wanted us to refrain from disturbing this planet. It wanted us not to take it over.”
The theme of Asimov’s story is that the misuse of technology and the abuse of the planet’s natural environment would inevitably lead to man’s demise. Asimov knew enough about astronomy to understand that the long-term scenarios depicted in his stories, especially in "Hallucination," were too far removed from the present reality to provide a panacea.