The reason is that life is so fragile. You can’t be sure if you’ll be alive tomorrow, so why not enjoy a moment of distraction and happiness as everything else were trivial and unimportant. The only thing important here, now, is the moment that this time doesn’t seem to escape because you’re living it.
A mere song has the power to disconnect you from this world and transport you to another galaxy of ineffable emotions and unsuspected feelings. So, happiness for me right now is a song, and anything else. Al least, I could say that I live in this world not worried about anything else, just close my eyes and sing this song.