The truth is that the word happiness is difficult to define as a concept since it can have a different meaning for each person.
That is true, it is very personal, and even when my friend asked yesterday what is the key to being happy and having a balanced life, it was difficult to respond to her immediately. We feel when we are happy but in those moments we live it, we experience it and not that much think what exact thing made us happy :))
Thank you for your excellence in this entry, the song is so sad and so happy at the same time :)