Stress
It depends on what you consider stress. Odds are, you're not being held up at gunpoint much (if you are, perhaps you ought to consider different source material)...but you are on a perpetual deadline to create funny strips. Daily, in fact.
And as mentioned earlier, comic strips are an art form, which means cartoonists are plagued by some of the same torments as other artists. For example, Charles Schultz drew Peanuts for fifty years and it brought him stupendous commercial success. Even Watterson considers it the most pivotal and inspirational strip of all time.
Schultz himself was kind of a tragic man, though. He was tormented with self-doubt, and in the end was so tied to his characters that he died the day after the final Peanuts strip appeared in the papers.