Stress
If you're lucky enough to drive every day, you know how stressful traffic and bad drivers can be. If you drive a taxi for a living, you can take that stress and multiply it by a factor of about a bazillion.
Being a taxi driver can be incredibly stressful work. Long hours, lack of (bodily) motion, back problems, traffic, fare evasion, passenger threats, and appallingly small tips can take a toll on a person. Some taxi drivers work up to eighty hours a week to make ends meet—and eighty hours a week of those stressors is enough to drive a person crazy.
Plus, all that coffee can't be great for your nerves, either.