Isolating the cause of the perversion is akin to panning for gold - laborious and challenging. And similarly defeating. I fully suspect as well that the offending person is convinced of his or her own ineffable propriety. They would likely consider the possibility of being untrustworthy as non-existent. Being untrustworthy is a disease which primarily affects others not the host carrier. By comparison pure mendacity (an outright lie) is more easily discoverable because it is prompted by a conscious purpose not merely a genetic condition. This however only makes the endurance of the malfeasance more agonizing - there is no direct relationship to thought or intent other than inspiring a lack of confidence. As a result the erosion of trust is potentially insignificant. It is mainly a consequence without an agenda - as tolerable as a child absorbed in a harmless fantasy. Nonetheless if one prefers to deal with adults the capitulation is both annoying and shallow. There comes a point when the manifest absurdity of the conduct is unfulfilling for at least one of the parties. Calling out the suspicion can be especially damaging for the culprit who hasn’t any concept of the mutuality of a healthy relationship. Their mistrustful behaviour may be for them reasonable protection and maintenance for their own survival (which for them is threatened). On a less conciliatory level they may be characterized as unduly selfish or at least self-absorbed neither of which particularly disqualifies the initial offence. Sometimes children need to be left alone to play - sadly a ruinous collateral to an otherwise well intentioned association. There is no advantage to the pursuit of a relationship not founded on trust. At best it will translate into a shift of focus from the purpose of one or the other but there will be no congruity. The competition is between two elemental forces, each founded on base necessity; and while they may coexist for a time, they will inevitably diverge. When that happens there is no amount of persuasion that will alter what was from the beginning instinctive. The relationship - like an overlapping Venn diagram - may have its limited mutuality but predominantly the disjunction is forever maintained. To engage in argument or observation about differences is guaranteed only to be hurtful on both sides. Unfortunately even polite confession of what is meant to be strictly exculpatory amounts to unwitting extension of deceit and therefore hardens the case for mistrust.
"In all thy humours, whether grave or mellow, Thou’rt such a touchy, testy, pleasant fellow; Hast so much wit, and mirth, and spleen about thee, There is no living with thee, nor without thee."
Addison, Joseph. “Essays and Tales”