toAnswer:
I think Mark Watney's sense of humor was one of the most primal characteristics which kept him sane through his ordeal on Mars. The thing is it is unbelievably difficult to be alone if you cannot enjoy your own company. I do not know the best way to explain this but as you know he was under a lot of stress and struggling to keep himself alive every moment. I cannot state the odds but most people tend to take the easy way out in these situations but Watney with his positive attitude and approach tackled one problem at a time.
That is what is special about Astronauts according to me. I think that astronauts are the most professional people. What I mean is that you wouldn't have six people on the Hermes in a mission like Ares 3 who will rub off against each other. The astronauts are selected based on their responses and reactions to various human stimuli and isolation existence. They will have had a lot of sociological evaluations beforehand to gauge their emotional stability and their attitude under duress.
Mark Watney through his positive attitude keeps himself focused. The moments depicting his frustration and agony are limited in the book and the movie but it exists. It is this ability of his to overcome his misery and have an objective approach at his fate that makes the apt astronaut.
One of the most striking examples is made available in the book. Watney always stresses letting the crew know that it wasn't their fault in leaving him on Mars and considering him dead. The astronauts don't always act on their instinctive emotions and that is their greatest quality.
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