Training to be an astronaut is a textbook-full of stressful situations—literally, there's an actual book of this stuff (source). You'll undergo some of the most intense situations government scientists can devise, to see if you can handle even more intense situations when you're finally in space. This'll include survival training—learning how not to die in hazardous space situations is fairly important—plus how to maneuver in space and how to deal with weightlessness, launch forces, living in space for an extended period, and worse.
Stress can also arise from dealing with fellow human beings on the mission. Besides the ever-present stench of body odor, boredom in an environment where even simple tasks can become unreasonably difficult can cause conflict. Anyone who's been stuck in an all-day meeting will know what we're talking about.
But hey, astronauts are supposed to deal with stress better than us mere mortals. That's actually part of the test. There's at least one way to deal with the stress that comes from space station crewmates driving you crazy: take it out on Mission Control.
Houston, you have a problem.