Stress
After traveling two hundred days of the year and beating people up four nights a week, it doesn't matter how "fake" the show is. This job is stressful. The travel schedule is no joke. A professional wrestler might spend four consecutive nights in four different cities. Which means plane flights, bus rides, waiting in airport terminals, checking in to hotels, irregular meal times, and irregular sleeping hours. No family time, no backyard barbeques.
It's no wonder that heart attacks, drug abuse, and high blood pressure seem to hit pro wrestlers especially hard. Nearly half of now-deceased male wrestlers died of heart complications (source).