When you design a cryptographic system, you don't say "Believe me,
it's secure", but "I did everything I could to ensure that you don't
need to trust me". See for instance the Stamper
FAQ (an interesting service, by the way):
It would be very easy for
me to try and say what a reliable and trustworthy sort of a fellow I am
[...]. This would not be good enough!!!
Compare with politics, in which candidates for any sort of political office are precisely trying to convince you that they are reliable and trustworthy... Imagine how it would be if there was a politician whose main electoral promise was total transparency to ensure that you don't need to trust him...