All But a Stranger
Thor couldn't understand.
Loki had always been complicated and it was difficult to understand his meanings. Thor had always known this about his brother, the God of lies, but still he had hoped he would gain awareness of his motives. He hoped to know why Loki had become murderous, to why he lied more often and to why he wouldn't let Thor help him. He was clueless to his brother's thoughts and to why everything he tried could not reach them.
Thor couldn't understand why his brother would not listen.
He had offered Loki a helping hand whenever he could. He had urged him to return to Asgard with Thor, to their home. He had tried to convince his brother that what he was doing would not help him or change anything. He had told Loki that no matter how many people died or how many suffered nothing good wold come from his plans. All in all, he had tried to help Loki to see what was right and to move him away towards that path. But nothing Thor did seemed to work, he couldn't change Loki's ways. He couldn't make Loki listen.
Thor couldn't understand how his brother was thinking.
At many points he thought that he had Loki figured out. That he had pinpointed what Loki had been thinking. He had believed to finally reach his brother in ways he never could before. On many occasions Thor had thought all of it would finally end, the suffering, the madness. Then only to have how wrong he was pushed into his face and Loki's smirk laughing at his foolishness. To be proven incorrect after allowing himself the joy of understanding his brother. To fall into misery after another failure. To know that he couldn't understand what images and views swirled in Loki's mind. These times are when Thor would realise that he did not even know how to decode his brother's doings.
Thor couldn't understand what his brother's actions meant.
Thor did not know why Loki did the things he did. He knew Loki was intelligent enough to know that his actions would gain him nothing. He knew that Loki must know that the manner he went about to achieve his means was not logical and would never work. So why did Loki continue his actions. Why did he do what he did? Thor could never understand.
Other times he would be tricked by Loki's plans. Thor would believe that he had deciphered his brother's next moves and hoped to prevent them. Only to fail as he was mocked for his stupidity and yet again his brother bested him with brains. Loki's cunning and quick wit had fooled Thor so many times they both had lost count. So even then, Thor didn't know his brother's actions.
And after Thor had reflected on all this, he had realised, that he didn't understand or even know as much of his brother as he thought he had. In their youth as they had separated and become different people, through years of Loki's suffering and Thor's joy. So the man Thor called brother could have been nothing but a stranger to him.