I think Gaius, like so many of us, like myself, wants to believe in something. You're stuck in a spaceship, and you've been there for four years, and you're just sitting here in a tin can, you would really hope that there was something else. And that sustained hope might actually keep you alive. Because depression is an illness. And if it gets too bad, then you're irrevocably damaged and there's nothing to live for, and there's nothing to hope for.
It's somehow very comforting to know that he considers depression an illness. It sounds like he understands the nature of it quite well. I feel strangely validated by this.
I have many things to live for. One of them is James.