Why do some people think their religion is better than others?

Question

and i dont mean cases where someone just decided it was the best religion for them after studying a bunch of them, i mean people who flat out say "this is what is right, and anybody else is wrong"?

i am not hanging out with the wrong crowd, i hear this quite frequently on here as well as from random people i encounter each week. im just mystified how a person can claim their religion as THE right one, if that is all they have ever known.