From The Mind and the Mirror by Dr. H. D. Charm; published spring 1380.
Almost as great as the common misperception about the nature of changeling intelligence is the lack of true understanding as to the full purview of changeling feeding and parasitism. While it's true that changelings can sustain themselves on love drained from other creatures, the monomaniacal focus on that emotion attributed to them by outsiders is a legacy of the reign of Queen Chrysalis rather than being the totality of that ability. Chrysalis, in her role as guardian against external threats by the conscious races, urged the changelings she controlled into a specialization in collecting love, as she found it the most available—and, with spells to induce comatose idyllic dreams in captured ponies, sustainable—emotion in Equestria.
In truth, even common changelings can potentially sustain themselves on the presence of any strong, sustained emotion. This is not precisely "feeding" as most other races understand it. Like dragons, changelings are sustained by a powerful magical core. Unlike dragons, they fuel that core not by consuming magically conductive objects like gems, but instead by gathering the ambient willpower that leaks into a creature's surroundings during a strong display of emotion and then converting it directly into stored magical energy. Changelings can eat normal food—but they can no more survive on it alone in the long term than a normal unicorn could survive entirely on magical energy.
This "feeding" process, in its most basic form, is harmless to the creatures around a changeling. However, the temporary magical links created by it can induce strange dreams or secondary sensory phenomena and even actively drain the power out of ambient magical fields. This association with dreams in particular led to an association of changelings with Princess Luna in the early days of the pre-Equestrian and Equestrian kingdoms; certain traditional art even depicts Nightmare Moon as somehow an ascendant changeling noble gone mad with power.