Let's say its a sunny day, 90 degrees hot and humid. And you see a guy walking around in a rain coat and he's sweating so much he's getting wet. You say to him, "That coat isn't working, its not going to keep you dry, its actually making you get wet. The best way for you to dry off is to take off the coat." He looks at you like you're crazy and says, "I'm not trying to be dry, I'm trying to sweat off five pounds."
Once you know the guy is trying to lose weight, then you could suggest better ways to do it. But if you assume you know what he is trying to do, you could give him advice that will be unrelated to what he was trying to do. Sure, it might be great advice--the best--but if it doesn't help him toward his goal then it isn't the best advice for him at that time. Also, it could ignore his goal altogether, which invalidates his intentions.
I'm just thinking, sometimes you have to ask someone why they are doing what they are doing to determine if what they are doing is successful.