What fundamental change occurs in Jonas as their difficult journey continues and they grow weaker?

At the start of Chapter Twenty-three, Jonas specifically experiences hopelessness for the first time. His strength is dwindling, the cold is taking over his body, and just for a moment he considers how easy it would be to succumb to the numbness he feels. Fortunately, love emerges victorious, in that Jonas pulls himself together in an effort to keep Gabe alive. He isn't so much fighting for himself, as he is for the baby he loves.