People always think something's all true.
Just like the line “People never notice anything,” this is an example of one of Holden’s extreme generalizations of other people’s behavior. If you pay attention to these, you will find that not only are many of them untrue, but some can even be applied to Holden himself. These sentences reinforce Holden’s character as an unreliable narrator.
To help improve the meaning of these lyrics, visit “The Catcher in the Rye (Chap. 2)” by J.D. Salinger and leave a comment on the lyrics box