Stress
Think of what it must be like to cycle sixty miles a day at top speeds in terrible traffic. You'll face rain, snow, and slick conditions with cabs gunning for you and people opening up car doors on you—all in the hopes that you can deliver a package on time. If you can't, you'll probably lose your job.
Also, imagine how many jobs you need to complete just to be able to stay in the business and afford food and rent. Now you're probably getting an idea of how incredibly stressful this job is.