"I did not fear my own powers!" Thor objected, his tone far less certain than his actual words.
"Mayhap it was just the rest of Asgard who feared thunder then," Loki huffed, his brows knitting into a furrowed V of displeasure as he recalled their early teens.
He'd already been training in magic for a number of years at that point, but Thor had not come into his powers yet. It was a brief window during which Loki had been able to lord his abilities over his slightly elder brother. Sure, Thor had already been the physically superior of the two in most ways, but at least Loki'd had his own advantage. That didn't last.
The summer of their thirteenth year, the storms came early. Or so everyone believed at first. Thor and his little brother had been bickering as they frequently did. In between squabbles, they were inseparable, like most siblings at that age. Training as warriors had commenced in earnest that same season, both boys now of age to really learn to fight for Asgard's glory. No less than a shining example of skill and bravery was expected from both princes.
Thor was eager for this; Loki less so. If he'd had his way, he would have spent more time with Frigga or in the library studying magic. And so it was with sullen reluctance that Loki joined his brother in their daily drills and lessons. His foul mood was taken out on Thor. Loki always had know how best to needle and taunt Thor. Meanwhile, Thor had the typical troubles of youngsters that age: he was brash, moody, impulsive, arrogant.
Loki pushed and pushed until one day he pushed too far. Wanting desperately to make Thor lose his temper in front of Odin and make a fool of himself by granting Loki an easy victory, Loki instead found himself pelted with rain, wind whipping around them as they fought. Neither boy paid it much heed due to the already erratic patterns the weather had been displaying. An Asgardian warrior did not let a little rain slow him down.
First blood went to Loki, only further enraging Thor, who wasn't ready to yield yet. The nimble trickster laughed as he evaded one attack after another, making feints to keep Thor from gaining too much ground. He knew it infuriated Thor. He'd called Loki "a gnat" for his fighting style, but already Loki was learning that the best way to survive a fight was not to get hit.
Odin looked on as his sons faced off against each other, both muddy to the knee and soaked through. The spectacle was starting to draw a crowd, even in the rain. Around them now were cheers and hoots. It just fed their dedication to the fight. Neither wanted to lose in front of people they would one day rule. Their fragile teen egos had no place for that sort of humility.
Evading another swing of Thor's sword, Loki had grinned through the rivulets of water running down his face out of his dark hair. "Come on, Thor," he'd said, "aren't you even going to try to hit me?"
It had been the final straw for Thor, who roared and charged his brother. But before he could reach the already skipping out of reach Loki, lightning split the sky overhead in a blinding flash.
Loki remembered hearing a loud buzz and then someone screaming. When he woke up, he was laid out flat in the mud and every cell of his body hurt in a way that had been hitherto inconceivable. He didn't remember yielding; he wasn't even sure he'd been able to speak afterwards.
The fight was over. Not long after that, Odin bequeathed upon Thor the hammer Mjolnir. To Loki, who didn't understand its purpose, it seemed unfair, an insult added to his injury. Odin rarely explained himself, so it was more than a millennium before Loki had come to understand that Mjolnir hadn't been a reward for winning the fight, but a protection because of it. The hammer helped Thor focus his powers, control and direct them. It was the same way some sorcerers and witches used wands. Loki just hadn't been able to see past it being a weapon and a gift. Now he knew better. All those years he'd envied Thor when, if anything, Mjolnir had been a leash.
"I know I wasn't overly fond of what followed." That hadn't been the only lightning strike Loki had taken. They never felt any less horrible in his experience. It was something he and Banner had laughed about, in fact. Even the Hulk did not enjoy being electrocuted. Small common ground, but it was something.
Edits: mine | Title quote from "The Avengers" (2012, Marvel Studios)