The problem inherent to those statements is it implies that you don't believe what is being said. It sort of implies that you're rolling your eyes a bit.
"my car can go 210MPH on the highway" "prove it" "but that's illegal!" "so show me on a track in the desert where it is legal then" "that takes too much time, why can't you just take my word for it, why do you always have to nay-say everything i say?" "oooohkay..."
It's like this in diagnosing problems with people who've had severe trauma and don't know if they can trust you on your authority. You say "so what trauma have you experienced" and they after a bit will say "trauma x and y at age 9, trauma z at age 12"; you say "have you seen anyone about those traumas?" "no but i feel i've worked through them and i'm fine now"
to which you must say "Show me"; in other words... no you haven't. Statistically speaking you can't work through childhood trauma by age 22, because growing up and going through puberty and dealing with highschool and then college and dealing with the whole sex thing is also technically "traumatic" in the sense that it "Causes a high amount of stress on the mind and body" - you're rejecting the hypothesis that they've presented because of your knowledge of psychology and statistics therein.
The interesting thing, and the reason i bring it up, is that there are fringe cases in psychology (and even cars!) that may be able to cope with a powerful sense of will and self worth. But these are fringe for a reason, you can take those as you receive them. But everyone who has "dealt with it" on their own CLAIMS that they've fixed themselves by themselves. And that's where being skeptical comes in.
No one wants to be called a liar, or have it implied that they have no clue what they're talking about. there's a reason i am not going into clinical psychology... I don't want to constantly burst people's bubbles. that's why people who are actually getting the therapy that they need despise going to therapy. You're not supposed to enjoy it. So while i want to help people, i don't want to do it that way.
Phew. What a weird post this is. My main point is that people are defensive by nature, and want to be self sufficient, and asking them to back up what they are saying usually puts them on the defensive.
The reason scientists don't have this problem that often is that when someone says "gravity causes things to tend to fall toward the earth" no one says "prove it!"... because it's accepted as true. If a scientist says "Gravity implies gravitrons" then we say "show me" or "prove it"...