How come it's usual for him to be joyless?

He's just the "serious one" in his group, I suppose that's the out-of-universe, character design reason.

But in-universe... His childhood wasn't the greatest, as a result he had to take on a lot more responsibility when he was younger than most do. He had to worry about his father, and his younger brother, and that just sorta conditioned him to always worry about everyone. Now that he's in the military, he has squad mates to worry about, as well as his family back home. He's conditioned to be concerned, and that makes it hard to relax and be happy--his mind is always on the future and what perils it holds, what could go wrong next.
Perhaps joining the military wasn't the best career choice for someone like him, but it's also possible that the structured way it operates is oddly comforting to him, and he feels it's a way to open opportunities and learn skills that he would otherwise never have available to him.

Even after all these years, he's a bit in development character-wise, so I'm wording things a bit vaguely since it's not all fleshed out yet. But I do appreciate you asking, thank you!