Axel picked up piano. Spending home all the time gave him the opportunity to self teach himself.
He enjoys playing, he found that it helps him release emotions out. Which is where he most likely realized his feelings for Jupiter.
Rosie likes when Axel plays, she wants to learn too! But... she can't sit still long enough to learn, but that's okay. Axel figured that Rosie just wants to sit with him for a while.
He also enjoys playing for Jupiter.