Pokemon Centers are pretty magical, aren't they? You just go in, heal your team and you're done. It almost seems too good to be true -- it's not like you pay taxes out of the money you frisk from unconscious trainers after you defeat them, so whichever kind soul that's paying the staff and keeping the lights on is really doing you a solid. Thanks... Obama? But hell, you're like what, 10 years old in Pokemon games? Where would they even send medical bills for a 10 year-old?
Yeah. Your mom.
She's a real trooper. After dad walked out, she raised you all by herself. Never complaining, she bends to the societal decree that mandates "All boys leave the house some day. I saw it on TV." Mom sends you packing, far before your time, so you can live the life of a Pokemon prodigy. You're so spoiled that you don't think for a minute that all those hospital bills are arriving at her doorstep. Ever the dutiful parent, she keeps her mouth shut and doesn't burden you with her growing mountain of debt. It's not like you should feel bad on every step of your adventure -- your mom would want you to have the best possible experience. Just maybe reconsider doubling-back to the Pokemon Center every time your Charizard gets nicked by a Bidoof.
Lots of Pokemon evolve via experience, or with the right friendship level, or after being force-fed a shiny rock, but a select few evolve when you trade them. It's an odd, almost backwards phenomenon; you can get your Onix to level 99, but it'll only become a Steelix when you give it away to another trainer. It comes off as kind of a dick move, like your Pokemon will evolve for anyone but you.
But let's look closely at Machoke's transformation into Machamp.
Man, Machamp really hulks out, doesn't he? Not only does Machoke juice-up and perfect his sick 'do, it also sprouts two more beefy arms. Seriously, who's Machamp trying to impress, here? Well, it's you, dummy.
Imagine you were a Pokemon who spent every day with your trainer. They'd travel with you, battle with you and even charge thousands of dollars to their mom's credit card to pay for your medical bills. Now imagine after all that, your trainer passes you off to a stranger. You thought you were a friend, like beloved dog, but it turns out you were tradable property, like a professional athlete. Wouldn't you be scared to love again? What's to stop this new owner from trading you away once more? You'd probably do anything to impress your new trainer, up to and including altering your genetic structure, defying the laws of physics to create matter and therefore evolving into an entirely new being. Anything so you wouldn't ever be abandoned again. And they say Pokemon are the monsters.