Having experienced the war in Iraq already, Ryan had no desire whatsoever to return. Ryan is a patriot, but he had seen even at ground level that the American presence had been badly managed and that the civil war in Iraq could not be permanently refereed by the United States. For these reasons, it wasn't something that he voluntarily wanted to be part of again.
He had emotion about it, but Ryan's biggest concern was not for himself, it was for his family. When he came back from Iraq the first time, he found out how hard the dread of something happening to him had been on them. As he contemplated returning to Iraq, now he knew ahead of time what his family would be suffering with at home, and this weighed heavily on him.
His feeling for himself was just complete resignation to the unalterable fact that he was going back into uniform. It was clear that a deep sense of fatalism brought him quickly to this acceptance. It's a philosophy that holds that all events are subject to fate or inevitable predetermination. A variation of this is the belief that the events are specifically constructive in nature to bring about certain ends. As Ryan put it, "Whatever happens, happens for a reason. So there's a reason for this. I don't know it yet; I don't know what it is about. But whatever happens, so be it."
Nevertheless, even in the absence of fate, Ryan made it clear that he had an unquestionable feeling of obligation, not only to fulfill his eight year commitment, but also as an American citizen to answer his country's call of duty. "The nation calls on me again, you d@mn well know I'm going to do it. That's just the way I am and that's just the way the military is. I'm not going to fuss about it. I'm not going to try to get out of it and complain. [...] I'm going to do it and I'm going to do my job d@mn well just like I did the last time."
With determination and a smile, Ryan said, "Nobody can get me down. I'm Ryan." Then he expressed his usual tag line, now uttered with a burdened weariness: "I'm happy," which he followed with a more candid melancholy: "...for the most part."