Because everyone thinks they are right about everything. This is because to believe anything is, at least, to believe that it is true. Thus, every belief I have, when considered individually, I think is true. But when considering the set of my beliefs in its entirety, I acknowledge that there are likely some that are false. Necessarily, however, I don't know which beliefs they are; if I did, of course, I wouldn't believe them and thus they would not be part of my set of beliefs. In philosophy this is known as the paradox of the preface, and has to be confronted by everyone, not just Christians.