Thor humiliated Iron Man in an act of revenge for his betrayal during Marvel's superhero Civil War. Superheroes have always tended to go head-to-head. In fact, it's something of a tradition for heroes to fight when they first encounter one another - a tradition even honored by the MCU. Thor and Iron Man faced off in The Avengers, with Stark only lasting as long as he did because his armor was able to absorb the energy from Thor's lightning bolt.
The 2006 comic book Civil War event was Marvel's greatest superhero grudge match, sparked by the Super Human Registration Act. The superhero community split, with Captain America and other unregistered heroes branded outlaws. The conflict swiftly escalated, in large part because of Tony Stark's critical mistakes; he worked with the other superhero geniuses on his side to create what he considered a WMD, a clone of the Mighty Thor. It didn't go well, and this clone of Thor killed Goliath in one battle before being destroyed by Hercules in a rematch. The superhero Civil War happened at a time when the real Thor Odinson was dead, having ushered in the final Ragnarok. Needless to say, that death didn't last, and when Thor returned he was seriously unhappy to learn what Stark had done with genetic samples he had secretly obtained.
Iron Man confronted Thor when the Thunder God visited New Orleans in search of the spirits of the gods in Thor #3, by J. Michael Straczynski and Olivier Coipel. Stark was in charge of SHIELD at the time, responsible for enforcing the SHRA, and he attempted to persuade Thor to register. Thor let him talk for a while, his anger building, and it was only when thunder began to rumble and lightning flared across the heavens that Stark realized this wasn't going to be a friendly reunion. "You took my genetic code," Thor fumed, "and, without my permission, without my knowledge, used it to create an abomination -- an aberration -- an insult -- and this you told the world was me. You defiled my body, desecrated my trust, violated everything that I am." His words were like thunder - and then came the lightning.
Iron Man's repulsor blasts had absolutely no effect on Thor, so the armored Avenger decided to do this old-school - hand-to-hand. It was a foolish idea because Thor swatted him away with a single contemptuous blow of Mjolnir that caused so much damage it would take Iron Man's self-repair systems two minutes to get him back into action. Thor followed this up with an electromagnetic pulse that shut down every one of Stark's systems, tore his helmet off, and made clear there is a great difference between a God of Thunder and a mortal man in a metal suit. With Stark humbled, he left him in New Orleans, promising their conversation would continue later. "My armor's fried," Stark cried out, dejected. "How am I supposed to get back to base?" Thor didn't even turn round to tell him: "Walk."
Iron Man had grown too familiar with Thor, confident he knew what his friend was capable of. Suddenly he was reminded the Thunder God was a force of nature, lord of the storms, and he realized in all their past arguments and disputes his former friend had been holding back. The humiliation was total because there can be no doubt Stark didn't walk back to base; instead, he'll have called for backup, forcing him to admit to his people that they had been completely outclassed. He had learned just what it meant to cross a god - and would never make the mistake of underestimating Thor again.