Harry has decided, I'm sure correctly, that Quirrell's ability to flawlessly adopt any persona and simulate any intention for long stretches of time while simultaneously furthering his own (true) goals makes him impossible to trust. But if Harry buys into Quirrell's claims, Harry is equally capable of perpetual, undetectable duplicity and therefore equally unworthy of trust. And if he believes that's the case, it's just going to isolate him further, since he'll conclude that anyone who wants to be his friend is either irrational or overlooking the factors that make him an untrustworthy person. So Quirrell may have just convinced Harry that literally all of his relationships, present or future, are tantamount to a sort of deception. Good job!