Why do pregnant women and mothers think they're right?

Not all, but many pregnant women and mothers think they know everything about pregnancy. I see it on yahoo answers, online forums, and in person. Everywhere these women go around telling people what can or can't happen because they have or haven't experienced it. My first pregnancy had severe symptoms at conception, severely leaking breasts and nausea, the doctor said it's normal for some people. But those judgmental mothers and mothers-to-be want to tell anyone who says they have similar symptoms in early pregnancy that something is wrong with them or they aren't pregnant. Then they recite some book knowledge that doctors understand are not concrete! Why do some women act like such know-it-alls?