Why should you not trust?

A quote comes to mind for this question which can apparently be answered with it.

"Never trust the trust that never trusts" - SK (date unknown)

It means that you should not trust something or someone that you know their trust is not trustworthy. You'll find that once a person starts lying or being deceptive they will have to build around the lie. If you don't trust it in the first place you will see all the oddities with their actions and Words.