> I immediately become suspicious of anyone who is very certain of something

Me too, in almost every area of life. There's a reason it's called a conman: they are tricking your natural sense that confidence is connected to correctness.

But also, even when it isn't about conning you, how do people become certain of something? They ignored the evidence against whatever they are certain of.

People who actually know what they're talking about will always restrict the context and hedge their bets. Their explanation are tentative, filled with ifs and buts. They rarely say anything sweeping.