Let's consider the white waves as a form of bliss & the black sand as a form of sorrow. We will surely be happy playing with the waves, even we could forget that we can drift into the middle of the sea if we aren't careful enough. When the play time is over, we need to walk back to the mainland, we need to go home. Yes it's sadden because we have to stop, but isn't that better than being swept away and lost in the middle of the ocean? Who knows sadness could save our lives then? Because every thing needs balance, likewise happiness & sorrow in life.