You miss one very important thing in asking that question. Let me be very clear here. Even if the trainer is weak. Even if the trainer is cruel. Even if the trainer is the worst one you could ever imagine... It is still the better option to be bonded to such a trainer than to be without one.
There exists only one sort of Pokemon that do not wish a trainer. And that is those who are so powerful, so unchallengeable, that they can already do as they wish. The Legendaries are among this number, of course, but they are not the only ones. Have you ever heard of a wild Dragonite being captured? Or another Pokemon at the pinnacle of its strength showing itself before a trainer? I doubt it. For they are the rare few who do not need one.
Yet... even for some of those, gaining a trainer might be a better option, not for them, but for those around them. Many of the ones who become that strong also become dangerous, and their rampages may be the source of nightmares in all the Pokemon around them. Trainers are sometimes the only things holding back what could be a threat to humans and Pokemon alike for miles around.
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There exists only one sort of Pokemon that do not wish a trainer. And that is those who are so powerful, so unchallengeable, that they can already do as they wish. The Legendaries are among this number, of course, but they are not the only ones. Have you ever heard of a wild Dragonite being captured? Or another Pokemon at the pinnacle of its strength showing itself before a trainer? I doubt it. For they are the rare few who do not need one.
Yet... even for some of those, gaining a trainer might be a better option, not for them, but for those around them. Many of the ones who become that strong also become dangerous, and their rampages may be the source of nightmares in all the Pokemon around them. Trainers are sometimes the only things holding back what could be a threat to humans and Pokemon alike for miles around.