Understanding hearing
To know why a person decides to only hear is to know the context of the situation that takes place. In some situations the sounds being made are not important to the person receiving the sounds. Some examples being background noise and sounds not meant for someone.
Cases of hearing
In some cases, a person in conversation may decide to not understand what the other is saying. In other cases a person may decide to have media be playing in the background. What each of these cases have in common is that a person decides to either not care of the sounds, or to have it passively running for them to process. The stimulus of sound can have an effect on a person and can influence the way that they think.
Processing sounds is involuntary and is an automatic process the ears and brains do, but the way a person reacts to what they hear is made by their decisions.