What does his joy convey regarding the severity of the war and its impact on the civilian population?

Jonathan's greeting, "Happy Survival," which he embraces not just as a trend but moreover as a genuine expression of his joy to have survived the war - contains two revelations. The first is that Jonathan is an optimistic and constructive character who does not allow past adversity to affect his ability to function in the present. The second, more subtle, implication is that he has experienced truly awful, frightening and life-threatening things during the war. If mere survival is celebrated, than the danger must have been severe. These two interpretations do not contain an impossible contradiction, however, but instead suggest the two sides to Jonathan's character - his awareness of the world's danger, but persistence in the face of it nevertheless.