That's an interesting take, though I maintain that he doesn't have any motivation other than his personal ones, which are entirely selfish and not something that are, well, an obligation. While I see your point and I admit, I hadn't considered that before, I think it's still something that he could have chosen not to do - it's a convenient inroad to the story, though, so it serves its purpose pretty well in that regard.
I'm going to replay some RDR this weekend I think. I've been meaning to do so for ages. I think you're way off in saying that John never laughs, he definitely does, as I remember the sound of his laugh and thinking "damn, that guy has a dirty laugh", but I can't remember the scenes or context. I seem to recall him laughing a few times in the opening chapter though, before he goes down to Mexico. And possibly in Mexico as well. By the time he returns to the northern parts of the map, he's a much more embittered character as the pressure to deliver for the federales has increased, and that last section has always felt darker to me. But generally, I think that IV's humourous elements are always slightly tarnished by the gloom.
To be clear, I like bleak and I like gloom and I'm not averse to a depressing story, I just find that in IV it's too much, too bleak and filled with unsympathetic characters who have forced themselves into their own little depressions. The MacDrearies in particular... just, damn. So much sadness in one family.
Niko suffers more than most central protags, in games generally, but in GTA specifically, he just has the worst time. As highlighted before, the thing that should have turned out well for him, his relationship with Michelle, turned out to be utterly false and he was being used. So that, the biggest glimmer of hope and the chance for happiness, was yanked out from under him as soon as he started to get somewhere with her. His encounters with S&F characters are always tainted with something sad or bleak. He just never catches a break!
Side note: This is one of the most interesting discussions I've had on here in months, thanks.