Happy Planning～しあわせの総和～

The amount of happiness on the Earth is always kept constant, and becoming happy makes a lot of people unhappy. So says the cheerful childhood friend of main character. Protagonist loses track of her, but reunites with her at school. Her personality becomes grim as she does not want to steal other people's happiness. Is it possible to break out from this depressing theory and bring light to her heart? [From vndbreview]