If you are happy all the time, how do you realize that you are in fact happy. Because to be happy all the time would lead happiness to be normal and ordinary.
This would them imply that if you stopped being happy then this would be depression.
So the only way to balance this is to have sadness, struggles and challenges that make us appreciate what is true happiness.
The next time you are feeling down, remind yourself that without this suffering, you could not be happy.