I think that people who are sad tend to eat more chocolate than happy people, because they feel that eating the chocolate will make them feel better. They also may think it may make them feel better, because they enjoy it and it is sweet.
I found that there are studies that have proven that sad and depressed people tend to eat more chocolate and other candy, then someone who is happy most of the time. Just like doing a hobby that we love can make some feel better, chocolate makes others feel better. It's something that the people can almost rely on. The sweet taste of the chocolate basically fills the sad with happy, though only for a short time. The happiness doesn't truly last.