There are a multitude of factors, but the biggest one would be that they don't trust and/or believe me. As an abuse survivor and as someone with multiple invisible illnesses, it's important that those close to me trust me and believe me when I talk about these things and how they have affected me and continue to affect me to this day and will for the foreseeable future. The second someone talks about how my abuser was a nice person and they don't believe I was abused or they don't believe I suffer from the illnesses that I do my own trust in them goes away and I want them out of my life as quickly as possible.
There are different factors to consider here.
If it's someone I just met, not making adequate eye contact, talking too much, ultra opinionated, talks about others when their not around. These are clear indicators to get away from a new person quickly, before they try to infiltrate my life.
If it's someone I've known a while, something would likely need to be very wrong for me to stop trusting them, because my trust isn't earned easily.
Lies. When I perceive that the person doesn't say the truth anymore. I will be less prone to trust that person.
Trusting someone requires that to individual tells the truth at all times. If someone can look me in the face and say still lie to me without breaking a sweat, then I take that as the sure sign the individual is not to be trusted.
Some people may think that a friend's decision to not tell the truth might be to my own good. I however disagree with that. If you love me enough, you'd tell me the truth even if it is going to hurt.
Love knows no deceit.
I hardly trust anyone that has betrayed me before regardless of how little the betrayal is. If someone can betray you for little things, they can also betray you for big things
I also hardly trust anyone that I've seen betray someone else before. Anyone that can betray someone else, can also betray you. I also don't trust people that lie alot. The moment I discover that a person lies, I stop believing whatever they say, unless it's something I witness or someone else has confirmed
So betayal and lying are two of the things that can make me stop trusting anyone
I stopped trusting people once I stepped into the important world of drugs and business. I learned regarding the intricacies of the human condition and therefore the prices of misplacing trust in a very person/group. I even have been disappointed endlessly by friends, family, colleagues, instructors, mentors, and former partners. With all this experiential data, I still like better to trust folks. it's not healthy to defend oneself from the idea of trusting. One should be even handed in their application of trusting another with sensitive data.