Discussions and debates about how to be happy have been topics of conversation for centuries. Believing that unhappiness is self inflicted seems to be the sentiment that rules concepts of unhappiness.
However, the poster above resonates as being the most correct assessment of happy/unhappy. Being unhappy is negative. Negativity is not intelligent. It is so simple!
What is more amazing is that Buddha was saying the same thing thousands of years ago. Let go of the negative.
Today, try immediately releasing any negative thoughts that pop into your brain. See if you feel different. See if you feel happy.
Go in peace.