When Mycroft was just a child, he used to have bad dreams. He would wake up in the middle of the night, covered in sweat and terrified.
Sherlock would then climb to his bed and hold his hand till he calmed down. They would sleep the rest of the nigh together. Sherlock loved his big brother.
One night, after one of these episodes, their father came to their room. He was drunk and started shelling at them. - They were grown up men - he shouted - not little girls. He would fix them –
Mycroft was used to this and endured the beat up without a flinch. He was only ten.
Sherlock was five, and not familiarized with this. He burst out into tears, screaming for his brother's help. His skin turning red and then purple under the fist of his father. He wouldn't give up and his father would not stop beating him.
Sherlock was hospitalized that night, with a broken femur and several concussions to the head. He had fallen down the stairs.
Mycroft visited him the next day after school. His little brother wasn't speaking at him, and his eyes had dried out of tears. So they sat the rest of the day in silence.
When Mycroft was about to leave Sherlock looked at him and his eyes were full of sadness. – Please. Stay – he muttered.
Mycroft was too scared of his fathered to defy him. He shook his head and left.
After this Mycroft and Sherlock would grew apart. No one would hold Mycroft during his nightmares. And neither him nor his brother would ever share fillings again. Caring was not an advantage. They had both learned that.
Sherlock is now a grown man, as hid father wanted. He and Mycroft have learned how to stay apart from their weakness.
But Sherlock has now a roommate who, without realizing, has started to care about. And his roommate wakes up in the middle of the night covered in sweat and screaming.
Sherlock can hear him through the door. But he will not come inside the room to hold his hand. He will instead seat against the door shivering and holding his head, while a single tear wanders down his face.
John knows his friend is not sleeping well at night. That he is always obsessed with some new experiment. John doesn't know Sherlock is not sleeping because he wants to hold him through the night.