Sam has been told just last week he is damaged in ways even the angels can’t cure, he is coughing blood, he feels sick, he knows he is far from okay. I think at this point he also knew there is a chance his condition will get worse, and that the trials might kill him at the end. When he says ‘Maybe they won’t be the only ones’ I think he was talking about Dean; living an apple pie life, without being worried about demons, monsters, and his little brother. Sam, of course, is wrong here. Dean will never have real better life if Sam is dead.