Here's why:
* Common nouns: "Dictionary" is a common noun, meaning it refers to a general category of things (in this case, a book containing words and their definitions). Common nouns are not capitalized unless they are at the beginning of a sentence or part of a proper noun.
* Proper nouns: Proper nouns refer to specific people, places, or things. For example, "Oxford English Dictionary" is a proper noun because it refers to a specific dictionary. You would capitalize both "Oxford" and "Dictionary" in this case.
Examples:
* Incorrect: I looked up the word in the Dictionary.
* Correct: I looked up the word in the dictionary.
* Incorrect: The Oxford English Dictionary is a valuable resource.
* Correct: The Oxford English Dictionary is a valuable resource.
Remember, using correct capitalization helps make your writing clear and professional.