Music is very representational of peoples emotions and feelings and has been over the course of history. People have written music to share their feelings, thoughts, and to tell stories.
Blues originated from African slaves in the south. They wrote music and sang while they worked in the fields. They put their feelings of back breaking labor into words. During this time, their singing was called "field hollers" and served as the origination of blues music.
After the civil war many black men went on to work as musicians. The music they sang was very soulful and emotional and focused on the life of the typical black American. They sang about everyday life, including hard work, love, poverty, drinking, railroads, etc.
It has to do with the creative part of the brain which is also the part of the brain that controls emotions. It helps get out stress as well.
Most musicians express their personal feelings through the playings of riffs, or short melodic phrases, that make up larger sections of pieces. It is the playing of these riffs in their own style and using their own sense of rhythm;musicians can make them short or long, can play many of them after another, or just a few and then add new riffs or old ones, change them in any way they see fit. Usually riffs are solos performed by one musician but can be played by any number of musicians who can follow along and join in the accompaniment. The Blues, especially, lens itself to this personal expression.