You make me happy! Says one person to the other. What the person really means is "they like the feeling of joy they experience in the presence of the other." What this means is that, there is a difference between joy and happiness, and most people take one for the other even though they are two different expressions naturally representing two different notions.
Primarily, joy is a natural external contribution from our interaction with the elements within our environment or surroundings. These elements are other people (friends, family, others...), nature (animals, vegetation, etc...), and things (house, car, money, etc...). Because these elements bring joy to us, we can become full of joy meaning joyfull, in other words, joy flows from outside in. Being filled with joy or joyfull implies that we are a vessel in which joy can be poored to the fullest.
Since the flow of joy depends on each external element mentioned above, one can naturally understand why, when the element is removed, the feeling we experience it its absence is sadness. Joy is a temporary feeling that we can experience from various external sources.
Happiness is a state of contentment and fulfillment, meaning that it is something internal. If only we paid more attention to the daily expressions that we use on a regular basis, we would uncover their true secrets. There is a reason why we say "I am happy" and not "I am happyfull", or "full of happiness." Happiness has to do with the development and blossoming of our own and unique God-given talent. Being able to identify our natural talent, developing it, and making it blossom is the secret to happiness and fulfillment. A talent is something latent in each one of us. It is like a promise in the sense that, they must both be fulfilled. Once someone's talent is fulfilled, they cannot envisione themselve doing someting different in life or being someone else, regardless of how much they get paid, rich, poor, or anything between. This is when we stop being envious or jaleous of others. This is truly a state of contentment.
Someone who is genuinely happy and fulfilled can no longer take, but gives in abundance, even by their way of being alone. It is far to common to hear someone say that they met an individual whose presence was transformative, radiant, bright, inspirational, etc... Someone who is naturally and simply happy is very pleasant to be by, they are capable of giving joy simply by being present. It is like a mysterious power that they carry from within. The sad reality is that this power is given to each of us in a latent form. The choice to develop our individual talent is our responsibility and the noble purpose of our life on earth.