Thor knew Loki was very afraid of pain. In truth, there was no one in this world who was not afraid of pain. Pain interfered with a person's thoughts and made their movements lag. Even if someone did not fear the pain itself, they would fear those side effects, because on the battlefield, they were fatal.
But Asgard was a civilization that did not allow weakness. More precisely, its martial culture had doomed it so that no one could comfortably cry out in pain. Because if only you were hurting, perhaps only you would die, but if you cried out and disrupted others, everyone would die together. Everyone had to appear tough enough not to drag down their teammates.
In an atmosphere like that, being able to remain weak, to shrink back and cry out in pain, was actually a special privilege. Even if it was only in front of one person, or a few, it represented immense trust.
Thor himself could not remember how many times he had seen Loki's tears. Most of them were when they were young, when accidents always happened whenever the two of them joined forces to get into trouble. When Loki was hurt, he would cry, and Thor would not scold him for being weak like ordinary Asgardians. Instead, he would coax him while helping him hide it, telling everyone outside that he had caused all the trouble himself.
After they grew a little older, Loki began playing pranks aimed at Thor. Sometimes he tricked him into some place and trapped him there, and sometimes he had Vines hang him up. Thor's ridiculous state made Loki burst out laughing every time. Thor thought that was fine, because it was always better than watching Loki cry.
Later still, they had differences, and their relationship was