Why did Frank Loesser write Heart and Soul?

Heart & Soul: The Life and Music of Frank Loesser is a 2006 American documentary film about Guys and Dolls composer/lyricist Frank Loesser. Directed by Walter J. Gottlieb, it is credited as the first-ever TV documentary about Loesser and was produced by Gottlieb and associate producer James F. Cooney in co-operation with Frank Loesser Enterprises and his widow, Broadway actress and singer Jo Sullivan Loesser. The documentary debuted on Public Broadcasting Service (PBS) Public television stations nationwide in 2006 and ran throughout 2007. The film details the triumphant musical career of Loesser, who rose from humble New York City beginnings as the son of German Jewish immigrant parents to become the acclaimed composer/lyricist of 5 Broadway musicals and several Hollywood movie songs which became American pop standards, such as "Heart and Soul" (with Hoagy Carmichael), "On a Slow Boat to China", and "Baby, It's Cold Outside".