You have been taught to carefully consider your own assumptions, making it much less likely you assign false motives to other creatures. When using Sense Motive to detect deception or discern a secret message, as long as your expertise die roll result is not a 1, you don’t think a truthful statement contains deceptions or infer false information from a secret message even if you fail your check by 5 or more.

Whenever you interact with a creature under a mental effect for at least 1 minute, the GM attempts an automatic secret Sense Motive check for you without your expertise die. On a success, you learn about the mental effect. If you specifically request a Sense Motive check to sense mental effects, you can attempt a Sense Motive check as normal, including your expertise die, in addition to this automatic check.