I agree with lizatola, joy is a feeling that comes about in a place of peace within. I feel joy when I am feeling hope, faith and love. I feel it most when I can feel all of those things about and for myself. I always found it easy to feel joy for others but it felt strange to feel it for myself.
I also used to confuse joy and happiness thinking they were one in the same but I learned that happiness is based on what is happening around me. Joy is based on what is happening within me.