The reason that "to" is needed is because "listen" can not normally take an object on its own. "Listen" is an intransitive verb (a verb that does not take an object or that does not act upon an object). When you think of the act of listening, you are not doing anything to a particular object; rather, you are perceiving it. The "verb + object" pattern can only work with transitive verbs, like kick, pull, push, cook, etc. All of those verbs act upon an object, modifying it or moving it in some way. So, in order to give the verb "listen" the appearance of a transitive verb, it needs the preposition "to." "to" shows where the action is being directed as well. Without "to," the word "listen" has no direction and simply describes the act of perceiving any given sound. For example: A: "Listen! Do you hear that?" B: "Hear what?" The speaker (A) simply instructed the listener (B) to "listen!" B then understands that he should try to listen for some sort of sound, but he does not know what, because the speaker did not direct him toward a particular sound. Here is another example: A "Listen to that chirping sound! Do you hear it?" B: "Yes, I think so. What do you think it is?" In this example, A used "listen to" in order to direct B toward a particular sound. Does all of this make sense? I hope this clears things up for you. If anything is still unclear, please let me know.

"To" is a preposition that indicates aim or purpose in this example. Here is an excellent website to give you a breakdown: http://www.dictionary.com/browse/to Recommendation: don't get too caught up on rules (such as asking "why"?) Just accept it and enjoy the language. You'll learn a lot faster if you just relax.