I don't sing because I'm happy; I'm happy because I sing.

The line expresses a Jamesian idea often associated with the “James–Lange” view of emotion: feelings can follow from bodily action rather than precede it. Read this way, singing (an outward act with physiological and expressive components) is not merely a symptom of an inner state called happiness; it can be a cause that helps generate or intensify the feeling. The aphorism also captures a pragmatic ethic: if you want a different emotional life, begin with what you can do—adopt the posture, habits, and practices that embody the desired state. It’s a compact argument for mood-shaping through action rather than waiting for the “right” feeling to arrive.