After they started spending the night together they found out that listening to Tony play the piano was a quick way for Harry to calm down when trapped in a memory of the war. Learning to play it himself became a way for Harry to better manage his nightmares and trauma responses. The occlumency and legilimency lessons Harry received from Lord DiNozzo helped as well. Finally being able to order his thoughts and memories in a way that allowed him to acknowledge the war he fought in plus learning ways to relax when stress became a huge was a blessing for Harry. For a long time Tony would only hear Harry play when he had a bad day, but now, like himself, Harry just liked playing the piano.