And most people really don't like being confined to small places for long periods of time (ask any airline passenger in the middle economy seat). They also don't like being away from loved ones (ask any soldier). Or having their days and nights scrambled (as anyone who has worked overnights). Or cranky co-workers (ask anyone!).
All of those things will happen to the people selected for the journey to Mars.
NASA is trying to solve the problems with its Behavioral Health and Performance program (PDF), which focuses on three risks for long missions. (Spoiler alert: They're similar to problems workers face here on Earth.)
No. 1 is what Leveton calls the "B-med risk": "adverse cognitive or behavioral conditions that could arise." In other words, the risk that a crew member will freak out and do something crazy.
Risk two: bad teamwork. This is when crew members make errors because they don't get along with each other.
Risk three: sleep. That's right, of all the things NASA has to worry about for a Mars mission, making sure everyone gets their Z's is a big priority. Lack of sleep raises the risk of making mistakes due to fatigue, just like on Earth.
NASA is using "analogues" -- comparable situations on Earth -- to study solutions.
Another is HI-SEAS, short for Hawaii Space Exploration Analog and Simulation. Scientists volunteered to live in a habitat atop Hawaii's Mauna Loa volcano for eight months to see how the isolation would affect them.