Happiness is nothing more than a positive assessment of your future. Think about any moment of happiness (or sadness) and your thoughts...
The lottery winner is thinking about all the new, "better" things that will happen in his life. At some point what you can do with the money, just becomes the norm.
Paraplegic or loss of a loved one. Sadness about all the things you will NOT be able to do. These are usually factual, and healing or return to happiness comes as those things are replaced with new thoughts, new ideas for your future.
While we can't just decide to "think positive", understanding happiness as simply our thoughts that the future (which no one can predict) will be good offers the opportunity to understand that happiness is not a thing, and not something that "just happens".