Neither sadness nor happiness lasts forever.
Happiness knows its ups and downs as everything is impermanent.

We should never supplicate or hope for happiness to stay with us all the time--it's in knowing its wax and wane that we appreciate it more.

It's true that a temporary loss of happiness does not obliterate it as, with time, it bounces back on its own somehow--and this makes it more precious and endearing.  

There's so much sorrow in life, yet the saddest among us do have some uplifting moments when some joy sets in. This somehow brings some comfort.

Buddhism has a lot to say on this. As a Zen and Taoist-influenced person, I learn to acquire a non-choosing mind such that it can accept what life metes out at any point of time--the good or the bad, the pleasant and unpleasant, the safe or the threatening....
This is equanimous acceptance and has given both peace and joy