Because all happiness is. Temporary.
In my view, joy can be though of as a deep contentment and sense of peace that it is possible to feel even in miserable times. It is not easy to achieve this, and even if we do, it’s not necessarily permanent.
But happiness, just basic, comparatively superficial happiness, feels darned good. That’s why I have frequently made the mistake of burdening my happinesses with the impossible task of lasting longer than is reasonably possible. Then, when the moment is over, I sometimes get pouty inside and feel “unimpressed” by my good feeling because it was fleeting. This effect is exaggerated during times that don’t feel quite as smooth and easy as others.
It’s during those times, though, when it’s vital to allow and create, if possible, moments of internal and/or external smiles. Furthermore, one must let those moments be what they are without diminishing them because of their temporary nature. It may be basic, and it may just be another way of “living in the present”, but after realizing that I have been belittling my own moments of enjoyment, I wanted to reclaim their significance in my life by calling them embraceable.