How many of us have heard this phrase?
"We can't do that!"
Or even better...
"You can't do that!"
Almost every time, the person (or us) is perfectly capable of doing that, whatever "that" is. What we really mean is, "please oh please don't do that!" or "I'd really rather not do that". We say "can't". We mean "won't".
The trouble is that "can't" turns it into a dare. "You can't do that!" "Oh yeah? Watch me!". Whoops.
Telling someone they can't do something often has two effects:
- Makes you sound rather whiny
- Makes the person more likely to do it if only to prove that yes, they can
- Makes the person more defensive of his position
Take away the dare. Not "you can't do that!" but "here's why this should be done another way". You're much more likely to prevent "that" from happening (whatever it is) if you can provide an alternative rather than a dare.