With Pokémon, you can do anything!

After his abndonment by Damien - and in this canon, losing one trainer prior - he formed a very volatile, insecure attachment that he didn't know how to articulate. Charmeleon, or Charizard, didn't feel like Ash was keeping up with him. And, his time with Damien left an impression on him; leaving someone hanging and mistreating them is associated with love and attention. So, his "tough love" came through with Ash. He was the stronger link of the two, this was how he was to behave.

To a degree, he was angry at him for not being the trainer he could be. This whole plot thread could be interpreted platonically well enough, but I think it's extra juicy if this was how he was dealing with very conflicting feelings towards Ash at the time. No matter his size, Charizard was still a tween/teenager acting out, who'd grown too big for his own good, physically. He's temperamental, and immature.

After Charizard Chills, he mostly buried these feelings after gaining proper respect and appreciation for Ash, and realizing his behavior was wrong. There's a good chance this isn't even something he realized.

So, he went years without thinking about it. It was Charla, of all people - or Pokémon - who noticed the way he looked at, and talked about his trainer; especially once he became aware that Pokémon were on the table for Ash, after the Bayleef incident at the tail end of ARRepaved.

Charizard denied it at first, and then dismissed it as something not to be. But, his affections simmered to the surface a few years later.

When Ash pursued a Charizardite Y in Kalos, a year or so after his full adventure in the region, Charla had pushed him to finally speak up.

They found the Mega Stone and used it. Charizard's unresolved feelings cause the Mega Evolution to go haywire and, shocker, cause Charizard to go on a destructive rampage. Ash and his other partners manage to subdue him in a desperate, scalding battle, at least...!

Ash isn't really interested in the business of going through their disobedience era again, so he confronts Charizard for a heart-to-heart about what went wrong up there. Charizard, himself, has only one guess as to what it can be, so he comes clean. He tells Ash how he feels; how he's felt. If Ash wants to reject him, that's fine; they can get it over with. But he can't use Mega Evolution if they're hiding things from each other or stowing away regrets. At least, that's his hunch.

Ash is shocked, to say the least. But, he doesn't react with offense, or a particular answer yet, at all. He's flattered. It's not something he's ever thought about before! What is Charizard courtship even like?

Well, there is one thing worth mentionig — The Burning Battlefield.

A Charizard's courtship ritual comes in the form of a duel; the interested party challenges the other to a brawl. No moves. No special powers. Just a physical battle of wills where both fight to wrestle and subdue the other. If the challenger can defeat the challenged, then that typically, and ritualistically convinces them to accept being their mate.

(Charizard had to challenge Charla many times, before succeeding.)

Now, this sort of thing sounds right up Ash's alley. Throwing down to express your feelings; your guts, and termintation!? He accepts!

Ash and Charizard tussle and fight. Obviously, Ash doesn't stand a ghost of a chance in raw power. But, he's far mightier than his comparatively scrawny build lets on, and it's enough for a fun tussle.

In fact, it's so much fun that they almost forget about the stakes!

Naturally, Ash loses, in the end; bruised, and a little charred. But, he's sweating from the sweltering heat from the proximity to Charizard. His heart is racing. He's so in the moment, that well, Ash does what he does best, and uses actions, rather than words. He kisses the big fella.

"You can put another notch on your belt with this one, big guy, so you'd better be proud of it. You've caught an Ash!"

Peeks and their other companions celebrate the results, of course. And, they test Mega Evolution a second time. This time, Charizard has complete control of his faculties, and his incredible newfound power.

The Mega Evolution was a success!