Joy is a primitive human emotion that has been part of our consciousness for thousands of years. We all experience joy regardless of our beliefs or cultural background. However, these days it's value is sadly underestimated, so much so that for many people it takes significant events for joy to even register.
The Joy Principle is all about recognising the value of joy and how it affects our happiness and gives us strength.
It's not complicated, but it might be difficult. You see, the idea is that you should look for joy in the present moment, which can be hard. We lead busy lives, we're stressed and worried about what seems to be really important stuff, so the result is that joy may not seem relevant and is hidden from us.
A child's smile, hello from a stranger, the sun on our back or a lovely meal. These are joyful events and recognising them as such is the first step towards understanding The Joy Principle.
Of course we all have different things that bring us joy, that's how it's supposed to work. Having said that, the goal is to see the joy in simple things first as this will have greater benefits and help calibrate your joy receptors. Significant joyful events like falling in love or the birth of a child, are recognised by everyone without any effort. The joy you get from buying things is short lived, the general rule is, "you can't buy joy."