Stress
There are different kinds of stress. If you're the president of a Fortune 500 company, there's probably someone running into your office every five minutes, handing you laundry lists of "crisis" items. Those people spend their days putting out metaphorical fires. With welding, you work long hours doing often repetitive tasks, but you're starting literal fires. And a company president rarely melts his fingers off.
A welder must always be precise, accurate, and put safety first. Welders can very easily burn or seriously hurt themselves. This has a tendency to create stress.
You're also working under demanding deadlines. Often you have to work at night or outdoors (because you're on a construction project and it's hard to build a building indoors). No matter what the cause, though, stress can be overcome by practice, experience, and common sense.
So get a good night's sleep, refrain from drinking on the job (unless you decide you don't need limbs anymore), and do what your superiors and more experienced colleagues tell you to do. The job is stressful, but it's stress that can be managed.