Why would someone try convincing someone not to believe everyone?

Let's be realsies here, some people's mouths spew more BS than a rapid fire minigun. As much as we might WANT to believe half that crap, some of our internal bullsh!t detectors still work. That person is a rare hero, so to say...

Because some people lie. It's not good for self-preservation to go around believing everything you are told. Sometimes you meet people who are extremely gullible. If you are a good friend to them you will try to give them a heads-up in the hope that they won't get taken advantage of.