I've always held kind of an envy for human growth cycles. It's basically a continuous and gradual shift, changes that occur slowly enough that they're easy to adjust to. And it's consistent; it doesn't change a lot between individuals. I'm sure you'd probably disagree, but from my perspective it seems so simple and hassle-free.
It's so much different with Pokémon. Evolution is part of the natural course of events for us; at certain points in our lives, many of us will change our appearances drastically, and with the new bodies comes a host of physical differences which we pretty much have to adjust to on the fly. It may not look it, but it is generally fairly uncomfortable; your body becomes something it has no experience with, and you don't get any training period to get ready for it. Sure, we get stronger as a result, but knowing how to use that strength isn't part of the package.
And evolution itself is notoriously inconsistent. In the natural order of things, it would occur at pretty set periods in our lives, but that's usually only the case in very strict circumstances. Evolution can be accelerated - any trainer knows this, the more a Pokémon battles the faster it evolves. That's because fighting speeds the processes that lead to our evolution; basically, it tricks our bodies into thinking they're aging faster than they are. It's fortunate for us that there aren't any real detrimental effects because of it. Sure, a young Pokémon may be growing into an adult body before it really has an adult mind, but it's not really that much of an issue; a Pokémon under a trainer already has an accelerated maturity curve and often the bond of training further settles any problems associated with that, and most Pokémon would gladly trade time in those preparatory stages for a body that they knew wasn't going to give them any more unpleasant surprises.
Evolution can be restrained, too. Trainers know the time-honored technique of startling a Pokémon out of evolution, but in the wild it tends not to be quite so easy. A Pokémon often has to work to keep from evolving before they're ready, and for some being ready can be quite a long time. And a Pokémon that's secluded and doesn't get out and experience life grows much more slowly, constraining its evolution. This is something that happens not infrequently when a Pokémon really isn't mentally ready to evolve; they become reclusive hermits, ignoring the outside world just so they don't have to experience that change. Unfortunately, it usually doesn't help them prepare for the evolution, meaning they can stay like that for their entire lives if someone doesn't force them out.
But there's a certain subset of Pokémon for which the process of evolution is even more unusual. That's the subset that evolve using stones or other objects, who basically have some measure of control over their evolution.
Take, for example, my kin, the Nido families. From Nidoran to Nidorino or Nidorina, it's a process much like any other Pokémon, the product of natural aging and experience. But from there to Nidoking or Nidoqueen, our evolution is reliant on a catalyst, a Moon Stone. And it can happen literally any time after we reach that middle stage. Basically, once we've matured enough, we can evolve at any time we wish, just by having a Moon Stone handy. If you're in a den in the wild, there's generally no question of this; trained Nidos tend to need to rely on their trainers having access to them, but sometimes they wander off on their own to find them. Either way, there's still an element of personal control.
As you might imagine, there are some benefits and drawbacks to evolving from the middle to the final stage. Both stages grow in different ways and gain different natural techniques for battle, of which either group may be better suited for particular situations or environments. But even beyond that, Nidorinos and Nidorinas are basically a period of adolescence, a time where growth and experience is encouraged, freedom is greatly increased, and responsibility is pretty minimal. Their behavior and personalities reflect that; they're usually much wilder, more reckless and antagonistic, more carefree and self-centered. Nidokings and Nidoqueens, on the other hand, a lot more is expected of them. They're the adult, the ones in charge, and they're expected to have the maturity to keep control of themselves and others. Not that they can't be a little wild and rough, especially the Nidokings, for whom some bravado and tough attitude are rather prized by trainers. But they also have that added responsibility, and their bodies experience different influences which result in different behavior and personality.
