Distrusting someone simply because they distrust you is childish for sure as well as somewhat naive, but I don't see it as being that malicious. Some people are naturally distrusting of others, so doesn't take much (JG is a great example of this). Trusting someone predominantly because they trust you in return is the sort of trust farming that can easily become abuse of the system as doesn't show good judgement at all.
Trust system is all about trust farming lol.
It is probably not going to work now because I am writing this post but test it 1-2 months later.
Distrust everybody in your trust list. Most of them will distrust you back without asking why you did what you did.
No matter how flawed it may be, it does have some logic: You trust your own judgement, right? When someone distrusts you, it means they don't trust your judgment. Logically, you no longer trust his judgement either.
Of course, this doesn't take into account how the trust system is actually supposed to work, which, in my opinion, is too damn complicated to make sense for the average Joe.