He's not terrified of dying in space. He's terrified of being alone. The irony of the story is that he finds more companionship twelve light years from earth with an alien than he ever had on his own planet. Grace had no living family, no close friends and no romantic partner. He sees others cynically because he has a rather poor self image and had really bad interpersonal experiences in the past. He wants company but doesnt tolerate others, he pushes people away but is mortally horrified of being alone. His closest and more admired fellow humans for him are his students because they represent his idealized hope for humanity. There's a whole section about this in a book chapter. They have curiosity, love for science and a strong sense of hope for the future, in the middle of a planet that is dying. They resolve their conflicts in mere moments, their biggest concerns are what's for lunch and at the end of the day, they go back to their parents. Grace finds his peace, and personal story arc, not from personal sacrifice, but by finding true companionship and friendship. Accepting the complexities that come from letting others into your life.

This also why Weir always goes for the happy ending. Why should Grace die in the end? Because it is more dramatic? Or is it because we have been programmed for decades to accept doom and gloom as the default? Is it because corporate slop wants us to accept the fate of the planet for profits? Weir rebels in the middle of a cultural zeitgeist filled with doomerism and dystopias. Every story that's turned "mature and realistic" is actually code for characters always being serious, everyone dies in the end, and dark monotone browns. It's a childish understanding of human nature. Mature storytelling is realizing that humans make jokes during tense moments, most of our moral problems are emotional and not technical, and maybe for once the hero gets to live after fulfilling his arc.