While mostly glossed over, I believe there was some small nod to Watney being one of the psychological anchors of the original team (at least in the film, I don't recall if that was in the book). Him being an optimist that doesn't give up is just as important to his survival as his specific scientific skill sets that come into play. That said, I believe a lot of his sanity was preserved by hope. He believed in a non-negligible chance that he could be rescued. People that signed up and are living a situation where there's no real hope it will change might have a vastly different reaction over time. Then again, they would also have company, and that can make a huge difference.