"If you walk out that door, don't you ever come back!" Those were the last words John Winchester would say to Sam for a long time. He knew those words would drive Sam away. That was exactly what Sam needed. John Winchester had made a lot of mistakes in his life, more than he could ever count. He had hurt everyone who he had loved and had loved him in return. Mary never got the love she deserved. She had been such a great woman and mother to his two boys. She had always put up with his crap, even when he drank, or didn't go home that night. Mostly John Winchester had ruined his children and he knew it.
Dean had been the best son anyone could have asked for. He was loyal, put family first, always did what he was told, and always took care of Sam. The one good thing John Winchester had ever done was give Sam to Dean. John Winchester might have ruined Dean, but Sam was okay. All the things John Winchester did for Dean were to make him stronger, to be able to handle what was going to happen. John Winchester knew he had to treat Dean and Sam like soldiers, knew they had to be tough if they wanted to survive. That's what enlistment had taught him. Dean took to it right away, he was a natural. Sam was always different. Sam never wanted to train. He wanted Dean to take him to the library to do research or to stay with Bobby or to stay in a town until the end of the school year. John Winchester had always given Sam exactly what he thought he needed. John Winchester pushed him to work hard, to be tough, and to fight for everything. That is exactly why he told Sam to leave.
Sam had never wanted the life they lived. Sam wanted to go to college, meet a girl, get married, and have a couple kids. Sam would make something of himself: be a lawyer or a doctor. Sam would meet a nice girl at college or work or volunteering to help some kids. Sam would get married in a big church, he always did pray and try to stay good and all that. Sam would have kids, retire, and maybe see some grandkids before he died of old age. John Winchester knew the end for Dean. Dean would die on a hunt because one day John Winchester wouldn't be there. Dean would die believing in him and that's why Sam was the smarter kid.
John Winchester needed Sam to go to college because Dean's and Mary's hard work shouldn't go to waste. Mary gave her life for Sam and Dean gave his childhood. John Winchester never loved Sam more than Dean; he knew that Dean loved Sam most. Years later when Sam and Dean found him, he was angry. He wanted Sam to stay gone, but Sam would always come back because he needed Dean and Dean needed Sam.