Be happy at others' happiness and sad at their unhappiness.
We are very jealous and it is really hard to be truly happy at the "success" or happiness of others. We are happy for people as long as they just "below" us. But, if they become equal or greater than ourselves, our real colors come out.
If we can be truly happy at the happiness of others, especially the people who are close to us, we can never be unhappy. But, it is damn hard to do it.