It’s probably not surprising that sunshine is so often used as a metaphor for joy and positivity. Most life on Earth, including humanity, is dependent on the energy of our Sun.
So we describe people as “a ray of sunshine” or as having “a sunny disposition.” Stevie Wonder sang “you are the sunshine of my life“, while Morecambe and Wise often signed off at the end of their show with “Bring me Sunshine (in your smile)“
The flip-side of course is our use of rain and cloud metaphors. My mother’s fond of the phrase “a face like a wet weekend” and I’ve always liked Billy Bragg’s “a little black cloud in a dress.” (Must I Paint you a Picture).
And when we want to offer hope in bad times, we promise sunshine after the storm.
Be thou the rainbow in the storms of life. The evening beam that smiles the clouds away, and tints tomorrow with prophetic ray. — Lord Byron