It is the symbol of happiness.
Wal Mart.
Yellow is considered a happy colour, thus having a happy face, have a happy colour. :) hehe
Smiley faces are yellow because the sun is yellow and the sun makes us happy. Although, smiley faces aren't always yellow they can be all the colors. But, generally, they are yellow.
Because they are always smiling at you!!!!! Because they are faces and they are smiley