Thor POV.
Thor saw Odin go into what Loki called Jotunheim. He had been told that it was a terrifying, horrible place. Why was Odin going there? Odin, apparently saw Thor following him, and said, "Ah, Thor, did you really think you could sneak in without me noticing?"
"No, but why are you going to Jotunheim, Allfather?" Thor couldn't help asking. After all, didn't Odin tell Thor it was off limits? He even told Loki, but who would believe he would listen to Odin?
"Do you remember where Loki is from? Or have you forgotten your brother's true heritage?" Odin said, smiling.
"Of course, how could I forget?" Thor agreed. He was just a young child then, still a baby.
A fight rages around Jotunheim, both sides losing many people. But, as soon as The Allfather had killed the king, Laufey, the Frost Giants lost morale, and lost the battle. Odin, looking for spoils, came across a young Frost Giant baby. The Allfather then took the baby home, turned him human, named him Loki, and raised him as his own.
Thor then thought of the times Loki had been angry as a kid, like interfering with Mjolnir, causing the handle to be too short, or cutting off Sif's golden hair. But now he understood why Loki had acted this way. He was angry about all the attention Thor had gotten, while he had to stay in the shadows. Thor decided he would try to make up for all those hard times he made Loki feel lesser than him.