Life is too ironic.
It takes sadness to know what happiness is,
noise to appreciate silence,
and absence to value presence.
I found that quote by unknown shared by one of my friends on facebook. I think it makes sense. But if you think about it, it also make sense in vice-versa.
These are some examples i found:
Heat is the absence of Cold,while Evil is the absence of Goodness.
From this, I believe that just like yin and yang,
everything needs its opposite to exist by the balance they creates to each other.