In its 15 Seasons run, Supernatural has had many familiar elements that fans of the show are well aware of like Dean's inherent love for pie and his Baby, the running jokes about Sam's hair, etc. Another such trope is Dean's continued woes with Sam for "abandoning" his family and its hunting ways to go off to college years ago. As the series inches towards its finale, it is finally explaining details and providing backstories, like how Sam had been planning to join a college for years before he took the big step and Dean knew all about it.
Till now, all that was known is that Sam Winchester had a heated argument with his father when he was 19 and left behind a heartbroken Dean as well as his family's "saving people, hunting things" motto to join Stanford University to pursue of his dream of a career in law. In every season, there is either a flashback episode or the Winchester brothers refer to their childhood in some way. But while, Sam's disdain for hunting and the constant shifting from one town to another had been touched upon, his initial plans of going to college like any "normal" person and his family's reaction to it has been mentioned but hadn't received much attention, until now.
In the latest episode of Supernatural, "Drag Me Away From You," the Winchesters arrive at the Rooster's Sunrise Motel in Wadsworth to look into the apparent suicide of an old friend, Travis, who they saved way back in 1993 from a child-eating monster, Baba Yaga. The story frequently cuts to the past events, starting from when a 14-year-old Dean was dropped off at the motel along with his brother Sam (9) by their father, John Winchester who was either on his way to one of his hunts or followed a lead to track the Yellow-eyed demon.
While entering the motel and grumbling about how his father should've let him tag along, Dean discovered Sam trying to hide a guide to American Colleges, which he had stolen from the last motel they had been staying at. Strangely, contrary to previous scenes of the Winchesters remembering their childhood, this time we see young Dean mocking an obviously dejected Sam and his dreams of going to a college. Earlier flashbacks of the young Winchesters showed the elder brother supporting and calming down an anxious Sam.
It's either because Dean was at an age where he was pretty impressionable, influenced by how his father saw the world and John Winchester had totally given up hope on ever having a happy, normal life. Or maybe it's a reflection of how Dean feels at the moment-- he knows Jack is going to die in his mission to kill God and nothing can be done to change it. His helpless and hopeless situation at present may have affected how he remembers this particular moment as young Dean's tone was derisive and he outright mocked Sam for daring to think that he has a choice, that he expects to have a life outside hunting and dreams of being accepted by a college when throughout his life all he has done is hopped from one school to another.
Thus this episode seemingly marks the exact time i.e., 1993 around which young Sam, fed up with killing things and yearning to have an "apple-pie life", had already started planning his way out. And the only exit from the life of hiding in the shadows, using stolen credit cards, and staying in rundown motels was to get into a good college and build his career, as well as his life, away from his family.