Happiness is a subjective phenomenon. It’s an opinion, a judgment, about one’s own state of mind. No one can say with any reasonable certainty that someone else is happy. Every person has to decide for him/herself whether or not it applies to them. If I think that I’m happy, then that is, by definition, happiness. There’s no outside way of proving or disproving my claim that I’m happy. The fact that I feel it makes it true.
If you could step inside my personal experience right now, feel what I’m feeling, you might decide that I’m a really happy guy based on your own standards of happiness. But to me, that same inner experlience might not qualify as happiness. But you would be just as right in calling it happiness as I would be in calling it unhappiness, because it’s a judgment, not an entity.
Can subjective phenomena be considered “real”? I’d say that would depend on the reality of the subject. In other words, is there really an “I” to be happy or unhappy? To whatever extent I am real, then my subjective judgments are real.