Conspiracy Theory: Some doors are benevolent spirits of dead adventurers.

A certain number of years ago, perhaps a few decades, perhaps a millennia, maybe it was last Thursday, something happened to a group of adventurers that ended up slaughtering them. The intensity of the experience caused a part of their immortal soul to be trapped in this realm. The piece of their soul that remains is imbedded into objects that dissuade others from coming into the same fate as them. Enter the Door...

A group of adventurers was hired to kill a witch who lives in a lone hut just over the hill. The mission is pretty straight forward: find the hut, find the witch, kill the witch. The only thing that stands in their way once they find the hut is the Door. Well, some time in the past, the witch was successful in fending off another, similar attack. The soul of one of the fallen party members is inside the door... denying entry to the rogue trying to pick it, refusing to budge to the paladin trying to break it down, resisting the effects of the spells that the warlock uses to try to destroy it. The Door-rior stands strong, not to protect the witch inside from those outside, but to protect the heroes from the horror that waits for them inside the hut. Eventually, as all things do, the door breaks, fails, and "dies" to the adventuring party who was ultimately more stubborn than the door was. This is, obviously, just the beginning of the battle for the victorious adventurers as they not will have to fight for their lives, or else become yet another lost spirit to occupy the next door the witch puts up.