Sherlock and John had very, very different childhoods. They grew up in different parts of England, with different surroundings and completely different upbringings. We'll start with John.
John grew up with his mother, father, and sister. He was older than Harry, his sister, and from the start they didn't get along. He had a fairly normal childhood, except the parts when his father was an alcoholic. John didn't know until he was about 13-years-old, and since then he hadn't really spoken to his father, but his father never made any particular effort to talk to him.
They were middle-class people. His father went from job to job trying to keep food on the table, and for the most part it worked. His mother worked two jobs, but she was still there.
He had a great relationship with his mother. She stood behind him in everything, she never loved him any less for anything he did or is. John always thought, and still thinks to this day, that his mother is perfect. John loves his mother more than he's ever loved anyone in his life.
Yes, John and the rest of the Watson family was fairly normal, every family has problems and their own secrets, right? That's what John always assumed.
Sherlock's upbringing was entirely different. He lived in the richer, more private parts outside of London, with his mother, father, and Mycroft. Mycroft was born six years before he was, and Mycroft was never particularly thrilled about having a baby brother.
Sherlock's mother loved him more than anything. Well, Mycroft, too. But Sherlock was her baby. Sherlock was the light of her life. Sherlock, with his little brown curls and bright blue eyes that would look at you and make your heart melt.
And Sherlock was brilliant from the start. He was so smart and learned so quickly. Sherlock's earliest memories are memories of sitting in his mother and father's gigantic bed while his mother read to him, and then it turned to him reading to his mother.
And she was always so proud of him. "My brilliant little Sherlock." or "My perfect little prince Sherlock." Sometimes, when Sherlock feels down about himself, his mother's voice rings out to him that he was perfect in every way.
Sherlock's father was away for business a lot. He would pat Sherlock's head and leave in his private car.
And Sherlock waited patiently for his father to come back. He would ask his mother what day it was, and when she'd say, "Tuesday." Sherlock knew it was time for Daddy to come home.
But one Tuesday he didn't.
"Mycroft," little four-year-old Sherlock asked, "Are you sure today is Tuesday?"
"Yes, Sherlock. It's Tuesday. Don't worry. He'll be back, and he'll probably bring us a new surprise. It's the second Tuesday of the month."
They always got presents on the second Tuesday of the month.
But he didn't come back. And Sherlock's mother cried a lot. And every night she'd pull baby Sherlock out of his bed to sleep with her. He'd cuddle up next to Mummy and she'd stroke his hair. When he was sick she'd rub his belly, when he was sad she'd wipe away his tears, when he was happy she'd bask in the sunlight coming from his eyes.
Sherlock, as a small child, never realized Mycroft was jealous. He didn't seem jealous, more just annoyed with his little brother, as older siblings can be. Sometimes Mycroft would sleep with Mummy and Sherlock, but sometimes Mycroft held himself away from them and did everything on his own. They'd all be out in the yard, and instead of picked flowers with Sherlock and Mummy, Mycroft would pick worms a distance away from them.
And then Sherlock turned five and got on Mycroft's nerves a little too much. Sherlock would ask Mycroft to do things for him, mundane things like tie his shoes or fetch him a glass of water. Usually Mycroft did, but on this day he was particularly irritated and wanted Sherlock out of his hair.
Mycroft didn't mean to say it. He wanted to keep the secret for his mother forever, but he couldn't help it any longer.
"You're adopted, Sherlock!" Mycroft shouted.
"What does that mean?"
"It means Mummy is not your real mum, and your real mum didn't love you enough to keep you."
Sherlock was confused, but he shook it off. "That's ok, I like the Mummy I have just fine."
"No, Sherlock. She's not your Mum. She's my mum! Just go back to where you came from! Probably Mars, no doubt!"
"Stop shouting. I'm going to tell Mummy."
"She's not your Mummy!"
Sherlock began to cry. He ran back to the house as quickly as he could, and Mycroft ran to hide. Sherlock found his mother in her room, where she usually was.
"Sherlock, darling, what's the matter?"
"Mycroft," Sherlock sniffled, "Told me you're not my real mummy!"
She scooped up baby Sherlock in her arms. She held him close enough for him to hear her heart beating quickly. "There, there." She soothed, but her heart was still racing and it made Sherlock uneasy.
"Is he right, Mummy? Are you not mine?"
She took a deep breath. "He is right, Sherlock. I'm not your biological Mummy. That means you didn't come out of me. But my darling," She took Sherlock's chin in her hand, "I love you no less. You are my baby, all mine. I've never loved anyone more than I love you and Mycroft. I am your Mummy, and I always will be." She hugged him tight.
Sherlock never, and has never, felt more loved or special.
Two hours after Mycroft told Sherlock the secret, he came back into the house. He went to their mother's bedroom, where her and Sherlock were painting on the bed.
"Mother," Mycroft said, scared, "I'm sorry for telling Sherlock the secret-"
"Come here, Mycroft."
Mycroft thought he was in trouble. He gulped and went to the bed. Their mother placed her arms around Mycroft's shoulders and hugged him tight. "It's all ok, Mycroft. We're still a family, just the three of us. And I love you both equally."
"I'm sorry, Mother. And Sherlock, I'm sorry."
"It's ok, Mycroft." Sherlock hugged his big brother as tight as his little arms would let him.
Since then, that moment, Mycroft made it his goal to, no matter what, protect Sherlock. He'd do anything for Sherlock that Sherlock asked. He'd be the man of the house.
The rivalry between the Holmes boys started when Sherlock was eight. They don't remember how, they just remember that Sherlock began to be smarter than he was the day before. And Mycroft felt threatened. Sure, Sherlock was their mother's baby, but Mycroft was their mother's 'smart-boy'. She'd say things like, "Mycroft, my genius son." or "Mycroft, my brain. Sherlock, my heart." That's how it was, until Sherlock also became the brain.
So the rivalry began. They tried to out smart one another, which only made them smarter and more eager to learn. But no matter how much Mycroft tried, Sherlock remains their mother's little baby.
So, yeah. Sherlock and John grew up on opposite parts of the track. They were different, yet similar. One evening, John decided to ask Sherlock about his father.
"What about him?" Sherlock asked, defensively.
"I…just…what happened to him?"
"He left us."
"Really? Why?"
"What do you mean why?"
"Like, was he having an affair, was he…just selfish?"
"I don't know. I was four."
"You were four? Geez, Sherlock, I'm so sorry."
"It's all right. I don't remember any of it, except my mother cried a lot and I slept with her until I was, like, thirteen."
John looked confused, "Really?"
"Yes. I'm not proud."
"No, no, I think it's-"
"Weird, I know."
"Sweet."
Sherlock half smiled. "When I got older, I figured out that he was probably, in a way, jealous of Mycroft and I."
"What do you mean?"
"My mother always showed more affection toward Mycroft and I. If my father had a rough weekend at the office, but Mycroft or I were upset, she'd go to us before my father. Or she'd let one of us sleep with her, and my father would leave the bed to sleep elsewhere. I never thought anything of it until I got older and realized that was probably why."
"I'm sorry, Sherlock."
"It's all right. It wasn't my fault. I was just a child."
"You're absolutely right."
Sherlock sighed. "What about your father?"
"I don't know. I think he died while I was in Afghanistan."
"You think?"
"I hadn't talked to him since I was 13."
"Because he was an alcoholic?"
"Yeah, how did you-"
"Lucky guess."
"He was an awful man. But I didn't pick up any of his traits."
"I know, they all went to Harry."
"That's mean, Sherlock." John chuckled.
"But truthful. You know it." Sherlock laughed.
Sherlock and John knew neither of them were brought up in the most normal way, but now they had each other to be abnormal with.