Sadness is the key to making life happy. For instance, if one was happy all of the time, their life would be boring. If happiness was all you felt, there would be no such thing as happiness. Sadness makes people connect. If people weren't ever sad, you couldn't empathize with others. And if you can't empathize, you can't love others. Sadness is the key to making life happy.
If everyone were happy, you would have nothing to empathize about. Also, empathy and love two different things, and one is not contingent upon the other. In a state where everyone is happy, I assert that everyone also loves one another.