Indifference

What's the opposite of love? No, not hate - it's indifference

Pain is good, because it makes us realise that we're still alive, albeit barely. (Probably from here). That thumping heart may now hurt, pounding away, but it shows that we care. We were there last week, we'll be there next week. If it didn't hurt, we'd pack up and go home. Sadness and happiness are much the same, and result from our participation. The opposite would be absence and an altogether different phenomena: indifference.

We can only act out of joy or misery, and they come together. Without the highs, there'd be no lows. And without either, there'd be no action.