Scaly and hardened nipple area

Hello, why is my left nipple area hard and scaly? It's like it's cracked and also dark brown while my right nipple is fine and light brown but can see cracks/lines starting to appear right nipple area aswell. There's no pain or anything but it's very rough when i touch it and i've tried rubbing it with my hands but nothing happens and if i rub too hard/fast it starts to hurt a little. What's wrong with it and how do i make it normal again? Thank you. 24 Male. Picture: https://ibb.co/CK0VxHc