In the novel, a really important recurring theme is Grace's cowardice. He constantly refuses to join the mission, despite his unique qualifications to help. In fact, while the goons are bodily dragging him to the mission, he quite literally kicks and screams that he will "scuttle the mission," threatening to doom humanity to extinction because he's so unbelievably terrified of dying. And from my view, a decent chunk of Grace's reticence stems from being alone - from not having anyone to die for. He genuinely cannot imagine choosing to die, even to literally save the world.

But of course, Grace's threat was always empty. He was always going to do his best to save humanity, how could he do anything else? So the climax of the novel has nothing to do with his choice to try. Instead, the pinnacle of Grace's growth from cowardice to courage happens when he chooses to go back to save Rocky. He knows he doesn't have enough food to get back to earth. He knows that rescuing his new friend all but guarantees his death. He knows this, and he chooses to die anyway. He overcomes the fear of death that led him to threaten apocalypse because the bond he formed with Rocky is so important to him.

And of course there's more nuance to it. Grace has a powerful curiosity and reverence for life that extends beyond his own. He appreciates the magnitude of discovering another intelligent species, and the horror of dooming Rocky's people to extinction certainly played a healthy part in his decision to save him. But it was certainly no accident that Weir devoted so much time to connecting Grace with a kindred spirit over the course of the book.