Image taken from here
"All from Beloved's side is sweet
whatever He gives to you.
There is no bitter, if you knew
the secret how to taste."
I think this stanza is in parallel continuation with "Echo and Call" (secret of taste) and "I'll meet my master" (everything from Beloved's side is sweet).
As i read this stanza, i am thinking that man has been made as a neutral thinker. Like he has been given choice to go either towards negative(which he thinks is positive) or positive. We have been given a tool to taste and feel "bitterness" but at the same time we can sense "sweet" too.
Challenge is that you taste bitter (consider it as a tool to study sweetness) and convert that bitterness to sweetness... in your perception... and may be in reality :s
In any case, you have to observe dark to understand bright...