As I was contemplating infinity, I think I finally cracked the code on Pokemon.
What if Pokemon don't actually level up. What if the level is actually the Bond that Prof Oak insists you have with your team.
All pokemon have the same base states that they start with, but the more familiar the Pokemon is with you, the more proficient you are at dealing better attacks on the enemy, and how to doge. The closer you are with your Pokemon, the better the bond (level) and more effective you are at fighting.
Granted, Evolving does increase their overall effectiveness, but the bond is still there due to the experience you've had with your creature.
Thus, when a pokemon is stolen, they are next to worthless to the higher level bonds of actual trainers, and when you trade your pokemon, their bond still remains high with the new trainer, as they acknowledge the new trainer as a worthy companion from their old one, and in order to impress their old master, they try extra hard (thus "Boosted Exp") so that one day, they may show their old friend and companion that they were not a waste of time for anyone.