Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. For example: "I went to the store." ("to" shows the direction of the movement)
Examples of "to" as a preposition:
* Go to the park.
* I gave the book to John.
* She sat to the left of me.
Note: The word "to" can also be part of the infinitive form of a verb (e.g., "to run", "to eat"). However, in these cases, "to" is not acting as a preposition on its own. It's part of the infinitive phrase.