Listening is difficult when your response is already waiting at the door.

If your reply is already assembled, you are not listening; you are waiting. The composing takes attention, and attention is finite, so whatever the person is saying arrives partially at best. They can usually tell, and the conversation becomes two people transmitting.

Try letting go of the response until they have finished. It will feel risky, as though you might have nothing to say. What tends to happen is that you say something better, because it is aimed at what they actually said rather than at what you predicted halfway through.