When joy is near

Why is it that when joy is near,
some fear will rise to speak,
and suck the pleasure from the day,
as if the two are twins?
Perhaps they are as opposites,
along connecting lines,
for one will be at either side,
like two sides of a coin.
In symbiotic certainty they rise,
to grace the play,
of life's eternal rhythms;
as night becomes the day.