The fatty realizes he's a pathetic sack of shit (thus the suicide contemplation).  Completely lacking the ability to fix himself, he seeks an externality to do it for him.  That externality is the army.  The military will direct his life for him, force him to become fit.  For some people this type of thing works.  They need external structure.  They need hand-holding.   But at the end of the day if he can't fix himself nobody can fix him.  The best the army can do is show him the tools and teach him how to use them.  He's got to pick up the hammer and swing it for himself or he'll get nowhere.