Here's why:
* Noun: A noun is a word that names a person, place, thing, or idea.
* Subject: The noun "collection" is the subject of the sentence. It is what the sentence is about.
* Object: The noun "collection" is also the direct object of the verb "love". It is what the subject loves.
While "rock collection" is a phrase that describes the type of collection, it's "collection" that functions as the noun in this specific sentence.