After rewatching the pilot and listening to a 'fangirls' commentary this idea sprang to mind when they pointed out the question of just how did John escape from the fire? Well, this plot bunny assumes that he didn't and both of Dean and Sam's parents died that night, and what might have happened to the boys afterwards.
Dean and Sam, orphaned and with no living relatives to take them in, are separated. Sam is adopted by a loving family. But Dean is placed in foster care.
The older boy moved around from family to family, eventually ending up in an abusive home which he runs away from when he is 14 years old. Dean lives on the streets for two years until he ends up crossing paths with Bobby Singer while the older man is on a hunt. Dean helps him, and Bobby takes pity on the boy and takes him in, even though he is quite a handful. He starts training Dean to be a hunter, and by the time Dean is 24 he's hunting on his own and is considered one of the best.
Dean is hunting a demon in California when he ends up saving Sam from his burning apartment. The scene one Dean remembers far too well from his own nightmares of his mother dying exactly the same way as Sam's girlfriend. Sam wants to help Dean find the demon that killed his girlfriend, but Dean thinks the boy will just be a dead weight to him and tells him so.
Eventually Sam gets Dean to agree to let him come along, and during the course of their search for the demon, they learn more about each other and about their own pasts and how they are intertwined.