In short, a Mars crew will have to get along and work together for years in a close-quartered, highly dangerous environment beset with expected but unimagined unknowns and crises. Needless to say, these challenges would tax the most developed of us. In such an environment, crew compatibility becomes of absolutely paramount importance. Even if the space station has the latest in futuristic technology, if the crew members do not enjoy working together, the flight will be a miserable experience. I suggest there are two basic approaches, one of which is science.
Psychology is the study of behavior and the science with the most to say about compatibility.
However, there isn't even a widely recognized psychological test for compatibility. Usually, it's inferred from people's responses on psychological tests or scales. That is, if two people perform similarly on various psychological tests, we assume they are compatible. However, I would submit that it borders on arrogance to believe our science has advanced to the point of being able to predict something so subjectively personal. Needless to say, he took the assignment.