Also, things can be very bleak when we only think of things in terms of happy or sad.

Our emotions are all basically caused by changes in our expectations, and this makes being consistently happy almost impossible. As soon as we get used to being good, we start to expect things to keep being good, and they start seeming normal. And normal or expected doesn't make us happy.

There's lots of other positive emotions we could be focusing on instead - contentment, satisfaction, pride, wonder, etc. We tend to ignore these and focus on trying to be happy all the time.

With sadness it's the opposite, we lump all our negative emotions in and call them all sadness. So we can start to think of depression, disappointment, even indifference as just sadness. Which means there's lots of ways to be "sad", but only one way to be happy, and it's only ever temporary.