Due to the challenge evidenced in such a question, I contemplated about making an reply. A lot of rationalities sprang to mind, but after thinking over and over, I managed to conclude my answer to 3 points. I draw for self-expression, personal sensitivity and adventure.
"Self-expression" is our Creator's empowerment to everyone to the rights to opinion. What differentiates each individual is by how a person expresses him or herself. Having a vent to let off ones inner turmoil or delight is essential; and when that leads to a artistic creation of some type, it can be shared with other people.
"Personal sensitivity" is different from individual to individual. Much of a individual ethical, moral and environmental exposers determines his or her perception of the world. Some artists focus only limited disciplines of drawing while some others are more adventurous and experimental as to assimilate all sensations obtained. Inspiration fires up sensitivity whether it be something heard, felt or read.
"Adventure" prevents one from boredom - keeping their minds busy with imagination, events, incidents, possibilities and dreams.These mental activities not only keep us occupied for short periods of time, sometimes forever. But until these visions are materialized into a tangible product, they can't be documented and shared with others.
These are the reasons I am a cartoonist, how about you?