Sympathy Vs. Celebration

Why does it come naturally to people to sympathize with a stranger only when they hear of their misfortunes. Why do we never congratulate complete strangers on their happiness and achievements. Why don't we empathize when there is cause for celebration?
This in turn makes me wonder about the extent to which people are truly aware of the authenticity of their sympathy.  

We never utter the words: “I know of a complete stranger who leads a happy life. Let’s celebrate him.”

Love,

Fearless