Studies show that more trusting people are often more capable of telling when others are lying.
Which is the kind of Fridge Brilliance only found in Real Life. A person gets mistrustful if they get burnt one time too many - if you can see lies and deceptions coming, people will not get a chance to betray you, and therefore you will not become hurt, wary and eventually (possibly) paranoid.
The opposite could just as easily be argued if one sees 'being lied to' as the cause and then 'becoming more apprensive of potential lies' as the effect, rather than the reverse.
A more likely explanation is simply that people who are more trusting of others are typically those with good social skills. Which makes them better at spotting and interpreting body language, getting a rough idea on other people's moods and current mental state (stressed, agitated, relaxed, bored, etc.) - and all of that helps in figuring out if someone is telling the truth or lying through his teeth.
It could also be the other way: if you can't tell when people are lying, protecting yourself means assuming that everyone might be. Being good at telling when people are lying means that you don't need that, so you can be trusting.
It's quite likely that all three explanations contribute to the effect. Psychology!
How about, if you lie too many times, the only ones who will hang around you are those whom nobody trusts?
Could just as well say, if you lie too many times you'll get so good at it you can hang out with anyone. Both statements could be true too, in different cultures or places or even situations. Cause and effect is like that. It is why scientists spend such a lot of time obsessing about proof and checking and rechecking things.
This may be related to Good Is Not Soft, or rather the notion that what you see (or think you're seeing) and what you get aren't necessarily the same. In this case, that would translate to "being good and being mistrustful aren't mutually exclusive". Good people can be just as discerning and mistrustful as any hardened, gruffy character but they feel no obligation to wear their healthy sense of skepticism on their sleeve in plain view. At that point, all one has to do is keep their wits about them and observe. The good ones will prove themselves worthy of your charity while the bad ones will do otherwise on their own, more often than not.