Sunset only become good through the power of the elements of harmony not on her own free-will. And It shouldnt be hard to guess what Cozys backstory is. Think of it she wanted control over the school of friendship and to become the Empress of Friendship so perhaps she had a pretty tragic and lonely childhood like Starlight. She probably was neglected by her parents or didnt have any and no one wanted to be her friend. Being unloved for so long must have made her want to become loved by everyone and have more friends then anyone through any means necessary.