Themes in Design

Why must there always be a theme behind every aspect of art.  Who is to say one's art work/design must fit into a category?  Can one create for the sake of their own artistic expression?  Or must there always be guidelines that limits an artist's imagination in their process of design.  

By all means, i know there are certain rules to follow while trying to achieve optimum design.  Why must an artist/design always have a reason for placement of objects, or choice of color, etc.?  Is the phrase "I thought it looked good," not good enough for the viewers  that is always trying to make sense of what is being seen. Why can't the work exist for what it is?