Here's why:
* It lacks a subject. A sentence needs a subject (who or what is doing the action). In this case, we don't know who is listening.
* It lacks a complete verb. "Listen" is a verb, but it's in the infinitive form ("to listen"). It needs to be conjugated (e.g., "I listen") to be a complete verb.
To make it a sentence, you would need to add a subject and a conjugated verb:
* Example: "I listen to you."