Dean only leads by example, and is too hesitant to embrace his own squirrely power to really be a decent Slytherin.
Dean isn't an idiot, and he's really self-educated, but he is nowhere dedicated enough to knowledge to be in Ravenclaw.
Hufflepuff's are all about hard work, nose to the grindstone, toiling away for others, and that just does not fit Dean's personality.
And so Dean's a Gryffindor. He's an adventurer at heart, rather than a hero, but he's got what it takes to throw himself into a disaster and come out okay on the other side. Although he's not fearless, his daring attitude helps him match this outgoing house. His open and excited attitude is his chief trait to me.
The Scarf of Sexual Preference says: Gives Furries A Good Name