Pokemon, like all organic life (even though some Pokemon are not organic), are characterized by growth, specialization, and reproduction. One of the biggest elements of growth in the Pokemon world is the phenomenon of evolution, by which a Pokemon takes a form more suitable to its environment and circumstances. This is a much more simplified and rapid form of the same general idea as evolution in the real world. The point is, Pokemon evolve because that's what their environment demands of them. This is especially obvious because many Pokemon evolve naturally over time, as they grow, and take on more powers and more complex shapes and abilities.
So if evolution is driven by the natural environment, how can we have man-made evolutionary items, such as the Metal Coat?
Here's my theory.
Pokemon are different from animals in our world in about a hundred different ways, but humans are not. The humans depicted in Pokemon follow the same general principles of behavior, technology, government, and societal values as real humans (except for the Pokemon part). Which means that they, like we, have an enormous impact on their environment.
What I mean is, in the real world, there are countless examples of animal species going extinct, becoming endangered, or having their evolution drastically affected because of human behavior. What if, in the Pokemon world, the humans have also affected their environment in such a way that the environments those Pokemon need to force their evolution no longer exist?
Let's take Eevee for example. If there were no evolutionary stones (which are man-made objects), then Eevee would not evolve into its three stone-based evolved forms. There doesn't seem to be any other means of doing so. It COULD evolve into Leafeon or Glaceon though, but only in very specific environments. This also applies to Pokemon like Nosepass and Magneton.
So what the hell are evolutionary items, like the stones?
They're an attempt by humans to rectify destroying the environments. If the environment necessary for an Eevee to become a Flareon no longer exists, humans needed to devise a way to keep that evolutionary path open despite that. So the evolutionary stones were crafted in order to preserve the biodiversity and rectify destroying/altering the environment.
What about things like the Metal Coat, or the Razor Claw/Fang?
These are different. Items that aren't mimicking natural environments have a different purpose, which is to create evolutions that weren't originally there. This makes them sort of the opposite of the evolutionary stones, which preserve evolutions that could naturally exist before. Take Steelix for example. It resembles an Onix with metal grafted to its body through a man-made process. Weavile could very well be a Sneasel with Wolverine-like implants by means of the Razor Claw. In the real world, this is a more-extreme version of what scientists do when they transplant bio-luminescence from bacteria and fish into rats. Essentially, evolutionary items like the Metal Coat and Razor Claw are surgical modifications to pre-existing organisms to increase their strength by unnatural means.
And what of happiness?
Happiness seems to be very similar to trust. Pokemon are not just animals; they have very human-like emotions and intelligences. What if "happiness" based evolutions are those in which the Pokemon permits its trainer to execute an evolution by unnatural means so that the Pokemon can feel more comfortable in its new form?