Here's a breakdown of why it doesn't fit into the usual parts of speech categories:
* Noun: A noun is a person, place, thing, or idea (e.g., "dog," "school," "table," "love"). A sentence is not any of those.
* Verb: A verb is an action word (e.g., "run," "eat," "think"). A sentence itself doesn't perform an action.
* Adjective: An adjective describes a noun (e.g., "big," "red," "happy"). A sentence can't describe a noun.
* Adverb: An adverb describes a verb, adjective, or another adverb (e.g., "quickly," "very," "always"). A sentence doesn't describe other words.
* Pronoun: A pronoun replaces a noun (e.g., "he," "she," "it"). A sentence is not a replacement for anything.
* Preposition: A preposition shows the relationship between a noun or pronoun and another word in the sentence (e.g., "in," "on," "under"). A sentence doesn't establish relationships in this way.
* Conjunction: A conjunction connects words, phrases, or clauses (e.g., "and," "but," "or"). A sentence is a complete thought, not a connector.
* Interjection: An interjection expresses strong emotion (e.g., "Wow!" "Ouch!"). A sentence conveys information, not just emotion.
Think of it this way: A sentence is like a building made of bricks (words). We can analyze the bricks (words) to see what kind they are (their part of speech), but the building itself (the sentence) is a different structure altogether.