He really loved his mom.
It is hard to imagine, in our time where modern telecommunications become commonplace, the dire aches which come when we are worlds apart from our beloved ones. How to bare such yoke, I fail to fathom. Perhaps, though, people back then were simply used to it. For me, a child of this era, I could never.
The book compiled a plethora of his letters from childhood to adolescence, but significantly lacked his later ones, especially those composed during war time. My guess is not many remained; that, or he did not write to his mother as much, for he must be occupied by his duty. Or perhaps it was just because it was, well, war. One letter mentioned that some letters to his mother got destroyed. That might be the case.