My first thought was that John took them to the Grand Canyon for hunting-related reasons, but I kind of like the idea that he just wanted to take his kids on an honest holiday. This is only, what, three years after the fire? Sure he’s started hunting but say he finds himself approaching a point of no return, seeing what this life is doing to his family and to himself, but the vengeance has sunk so deep into his bones and he’s so angry all the time and he’s drinking too much but he loves his boys, god help him he loves his boys god help them, and sometimes he feels guilty and does things like buying them ice cream for no reason or, say, take them to the Grand Canyon. Like a real family. A real family on vacation, he thinks to himself. Imagine that. It’s a strange thought, and one that hurts.
Every time John does something like this, he thinks okay this is it, I’m gonna stop, this will stop, we’re gonna settle in some quiet town and move on with our lives, and he means it every time. He looks at his boys and he means it. Sam sits on his shoulders hugging John’s head and Dean scrambles ahead of them, eager to see everything but frequently hesitating, always looking back like he’s making sure his family’s still there, and his father nods, and Dean scrambles off again until he looks back again.
But then John thinks if only Mary were here and that’s always when things go downhill.
If we take it at face value when Dean claims he barely remembers the Grand Canyon, then no wonder, because what if the Grand Canyon was the last Dean saw of John being simply their father - not John the hunter, John the drunk, John who calls the shots - but simply John the man who loves them? John before hunting and revenge tarnished that love and buried Dean’s memory of the Grand Canyon under something less ephemeral, the more constant memory of loading a gun, the habit of pulling the trigger.
The memory becomes abstract. It withers until it is a memory of a memory at best.