Some people like to put themselves into a specific mood when they are doing something artistic. This might be something a musician, a painter or a writer does to help them to get in touch with their art.
People want their art to touch people and so if they are touched when they are creating it, they feel they will be able to more readily transfer this quality into their work. This can work with all sorts of emotions depending on how they want their audience to be touched.
So if they wanted to create a piece that represented something sad, then they might decide to work with sad love songs in the background. Or they may listen to them beforehand and work in silence if this is their usual method of working.
This would be similar to times when you are in a good mood because you have a good song in your head that keeps playing over and over again.