At the point he left, Serah had no fighting skills whatsoever. If I was Snow, capable of taking on huge tasks on my own, and Danielle was a wuss (like Serah was) and there was a team called NORA who were all capable of defending her, I'd probably want Danielle to stay back with her crew in a safer place. I guess.

But then, I'm not one to take on the world. Snow is, and Snow does as Snow does, and I can understand why he did it. You don't have say you'd do the same, but he was doing what he needed to do for his loved one as far as he was concerned. He didn't expect it to take years, obviously - that's a result of having to, as mentioned earlier, save the world.