What makes to hum a song when we are doing work?

In the olden days or in this modern era there are people who are at work sing a song. For example, when we go to the village the farmers who are cultivating the agricultural land use to sing songs so that they can forget the difficulty in doing the hard work. But Sometimes some people without knowing when they are doing work they used to hum a song with a slightest of lip movement. Is it because they are happy at their work? Is it because they want to forget the difficulty that they are doing? What is the reason behind this? Do you have a habit of humming your favorite song when you are at work? Share your thoughts.