No, wait, but isn't that reason for trusting him kind of nonsense? Because he's deciding whether or not to trust this strange girl claiming to be his best friend by deciding that he'll always trust his best friend. But his decision on whether or not she even is his best friend is based on whether he trusts her or not, which in turn...
Is based on whether she's his best friend or not, which is based on whether he trusts her or not, which is based on whether she's his best friend or not, which is based on whether he trusts her or not, which is based on whether she's his best friend or not, which is... &c. &c. &c.