After watching Harry almost die three times from cardiac failure after a brutal attack from assassins sent to kill him in order to get to his father, Zoro struggles with over-protectiveness. This has been eased due to the close bond he's formed with Killian, who came secretly to not only check on his son but also the Swordsman who had stolen his son's heart. The older pirates' similar fears and steadfast strength provide Zoro a harbor of refuge in which to work on the fear of almost losing a loved one again.
He realized he was utterly head over heels in love with Harry when he almost died in his arms and then his heart stopped beating for a few minutes while they tried to get him to the Going Merry.
Zoro changed one of his earings to a hook for Harry.