Why doesn't mr white want his wife to open the door?

Mr. White doesn't want his wife to open the door because he is afraid of what might happen if she does. He has just realized that the consequences of their wishes, granted by the monkey's paw, can be dire and lead to tragic outcomes. He fears that opening the door could bring in something connected to their last wish, which has already caused them immense grief. Ultimately, he wants to protect her from the Horror that might come with it.