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Bear in mind there are people in the Pokemon-verse who are either no longer trainers or never were in the first place. Some of them live and work with Pokemon in their daily life, and there's no reason why an equal footing for a relationship to form isn't practical.
While a trainer/mon relationship is subservient due to the nature of a competitive team, it would also be somewhat similar to a student falling for their teacher, a player falling for their coach, or vice versa.