Happiness & Misery:
Happiness and misery are the two legs of an individual: You need both to walk. If you choose happiness and reject misery, you will be limping. If you are obsessed about being happy all the time, that itself is enough to make you unhappy. If you decide that you want to be happy for one whole day, your mind will give you enough reasons to be unhappy.
If you want to be miserable, then make happiness your highest goal. If happiness is the only objective, then what about those moments when you are experiencing pain and suffering? For example, what happens when you stump your toe on a chair leg? What happens when your favorite team loses the game? What happens when you genuinely have to be sad? In all these moments, you go into an internal conflict because you have rejected all other emotions, and you only want to be happy. This, by itself, is enough to cause suffering.
Accepting all emotions of life and everything life can throw at you with equal-mindedness is the whole essence of being alive in the moment. Being alive in the moment is not to choose between happiness and sadness. If there is happiness, experience it; laugh and rejoice in that happiness. If there is pain and suffering; suffer in that suffering. Both pain and pleasure complement the way of life. You must accept both equally to be able to experience life in its completeness.