"Li ne estas feliĉa, alivorte li estas malĝoja."
Translation:He is not happy, in other words he is sad.
Is not being happy really other words be being sad? One could be angry instead of happy.
I think its not "not being happy", rather being unhappy, which translates to sad. I don't know if it depends on the culture, but it seems perfectly natural to me that unhappy/sad is the exact opposite of happy.