It’s true, but no one is quite sure why, especially given the fact that no other pokémon with stone-based evolutions have similar aversions. Even when the evolution in question wouldn’t result in a massive power boost (as in the case with jigglypuff and clefairy), if given the choice of whether or not to evolve, those pokémon would jump at the chance to do so. For that reason, scientists believe that it doesn’t have anything to do with the stone itself.
Given how common evolution is in the pokémon world, it’s unlikely that it’s due to the fear of evolving, either. Certainly, some pokémon have a sense of trepidation about evolution, given how drastically it alters their form (and sometimes, by extension, their minds), but at the same time, most pokémon see evolution the same way we humans see growing up and becoming an adult. When you’re a small child, you look forward to that change and all the freedoms it represents. As an adult, it’s entirely possible to become disenchanted, but the point is, when you’re a child, you very likely dream about what it would be like to be an adult. So to a lower stage pokémon, being fully evolved represents freedom and power, and many pokémon that can evolve dream of that … except pikachu.
Personally, my theory is that it has a lot to do with the pokémon that don’t see evolution as a pleasant experience. Pikachu is certainly rare in that the opinion is prevalent among their species, but there are certain individuals in every species that openly resist evolution. In many of these cases, these individuals simply see themselves as being perfect as they are. Thus, to them, evolution in part forces changes they don’t want on them, but more importantly, it also represents giving in and admitting that they feel they need to change. You could even say that resisting evolution is an act of rebellion—a way of showing later-stage evolutions that they can reach their potential without needing to change a thing about them.
As for eevee, the urge to resist evolution is actually rare because of that choice. To an eevee, evolution is a massive part of their identities; being an eevee is merely the introduction stage before they find out for themselves who they are. Some eevee know right away which evolution they wish to be, and thus, much of their time may be spent preparing for evolution. Others who don’t quite know take pride in the fact that they have a choice. They take their time deciding, and they value that choice highly. Certainly, there are a few individuals who find the abundance of choices highly intimidating, but these individuals are by and large rare. Overall, eevee generally find the choice itself to be both exciting and a vital part of being a member of their evolutionary family. A cultural identity, if you will.
