Remember when, back as a child, you were afraid of the dark? Afraid, that there might be a monster at night that’ll grab you? Remember how scary that was, even though you knew supernatural creatures did not exist?
Now imagine what it must’ve felt like for Dean as a kid, knowing that there were monsters lurking in the dark, who could attack you at any time, for real? Left alone by his father for days, having also to protect an even younger sibling, wielding weapons no child should have, against a threat he stands no chance against.
Imagine he had to do that while also reassuring Sam that nothing was wrong and pretending monsters didn’t exist.