"Entei, why humans?" Such a common inquery. The superior legendaries have asked that to me as often as any other Pokemon. One could shrug it off as easily as "why not," while one who may know history may point out that we've always done it that way. However, I find these justifications poor and ad hoc. Truthfully though, I've never had to really think about it. It was obvious to me, even at the very beginning; Even the worst that humanity has to offer is often better than the stagnation that we do.
When I've used to say this, only a select few ever got it. I am a terrible communicator, but it's also unconvincing on the face of it. Humans have been responsible for a lot of wrong, in both their own worlds, and even when they have been brought here. While we've tried our best when picking them, many humans are very rude and inconsiderate. So then what could possibly be so convincing?
The simple fact of the matter is that Pokemon are adaptable. It's what we're best at. Pokemon are able to change on a whim to suit whatever environment that arises, whether that's as mundane as a change in climate, such as the summit dwelling Alolan regionals, or as drastic as time itself freezing. Our adaptability lends itself well to settling into the worst that can be offered.
Humans aren't particularly adaptable. The slightest nudge and they're sent reeling, unable to balance themselves for what has been done. Of course, if that was all they did, they wouldn't be very useful. What makes humans so intriguing, and so valuable, is that they are able to work around adaptation. Their oft unwillingness to adapt, and to instead improvise and work hard to overcome the pressures that be, that is what makes them useful.
In most cases, Pokemon are willing to just live in whatever world they're stuck with. It makes for stable societies, something humans comment on every adventure, at least once, but unfortunately means that few are up to the task of trying to save the world when the situation calls for it. They would rather become entropy itself, than to counter it. Humans, however, always see room for improvement, refusing to keep their situation if they deem it as poor, and they're always investigating for the potential for a better future. If they see entropy, even if they cannot stop it, they kick and scream, never going quietly into the abyss.
There are exceptions to the rule, of course, on both sides, but the exceptions are just that. The exception.
Of course, using a human is never perfect. Humans still have the same emotions, and commit the same actions that make the rest of us fallible. Greed, envy, lust, loneliness, vengance, fumbles and corruption, such are the many examples I've seen over the years. I've grown to know them quite well over the many millennia of my life. Humanity has a way to portay them in the most destructive way one can bear witness to. It's rather quite impressive, the creativity employed by humans to right the perceived wrongs they see with the world. Some would argue this makes humanity unworthy of being our saviors, seeing them as doing more harm than good.
I vehemently oppose this position, even if such a criticism is with warrant. What they don't know is that the legendaries have to pick the humans. I've been to many worlds where the candidates where the living conditions were abysmal. Worlds destroyed by humans actions, with weapons far beyond their own comprehensions. Worlds destroyed by alternative versions of the legendary team, attempting to do what they thought was right. Worlds where the human spirit has seemingly been all but snuffed out. Many of these worlds only exist where humanity is barely able to eek an existence out. In all of these worlds, if they even existed, Pokemon simply adapted to the circumstances.
In spite of all of that, when given the chance, humanity still finds a way. Generosity, kindness, justice, trust, community, perseverance and integrity, such things and many more are just as possible as their negative counterparts. Just as their negative counterparts saw to it that our world was filled with hopelessness and cruelty, these positive counterparts wash away these blights, such actions reversing the damage caused by one. This is not to say the negative did not happen, but rather that the positive is meaningful enough to make up for it. Besides, we have our ways to avoid the negative outcomes.
Some may ask how we avoid the negative from happening in the first place. It's quite simple. Most humans candidates are lonely, and feel purposeless. Call it manipulation, or call it brilliant tactics, but we have consistently been able to give purpose to these humans by giving them a partner. These partners vary greatly in all but one detail. Some believe in humans, while others don't. Some are criminals, and others are already bounty hunters. No matter how different they all seem, sooner or later, they were all to hit a metaphorical wall in their own lives. These partners provide companionship, a tether for the human, and most of all, a purpose to fight for. It has not failed us yet, and I do not see it failing in the future.
One human, a long time ago, told me that the human spirit is indominable. I find myself having to concur with their assessment. While this is ineloquent to try to say aloud, infeasible to describe my thought process each time the question is proposed, I must always conclude that this is why humans.