Why was Charlie's relationship with Alice so difficult for both of them?

Alice is Charlie's teacher in his literature class for mentally disabled adults who is impressed by his hard work and motivation. She recommends him for the procedure originally, and oversees that he is doing well in the hands of the scientists throughout the book. She is the only woman who Charlie loves while he is intelligent, and it is only for a brief period. Charlie's fluctuating intelligence and emotional instability makes any relationship with him difficult. Alice must also balance her role as teacher with her role as Charlie's love interest.