Being different can mean having a unique perspective, but it can also mean being frustrated, having low self-esteem, and struggling to find environments where your differences don't dominate your life.
A design which gives you and everyone around you to smile. After all, a simple smile is what we need most of the times. Bring a bit of happiness in your life with this message of smile in a beautiful script.
The "sunshine" is "you". And "you" are the one that makes the singer happy. "You" are the one that gives the speaker meaning, and far as the speaker is concerned, "you" are the reason the sun shines at all.