Just as a painter needs to add tonal variety to a painting, so too must a musician add dynamic variety to a piece of music, it would be very dull otherwise.
In this video I look at a simple device to add these dynamics. It is not just about playing a crescendo or diminuendo though, it's also about tension and release.
The video doesn't tell the whole story... it is a very basic introduction to dynamics. Playing louder as the notes climb in pitch can be useful and, for example bring scales to life, but one extra thing I should mention is that when the volume increases as the notes climb, it can be very pleasing to the ear to hold back the volume on the highest note.
A simple exercise to develop your use of dynamics would be to play i and m alternating on the open top string, start softly and gradually build the volume and then relax it. The next stage would be to apply this to a scale and then to a phrase from a piece of music you are working on.