Hiya
I agree with Dani, we say no to our children to keep them safe from harm, give them guidlines & boundries, which they test as its human nature from a very early age!
Boys and some girls lol...do lash out in ways that alarm a parent, how many times have you seen a child throwing a temper tantrum in the supermarket cos the marketing people put the sweeties near the checkout to tempt them, and mummy or daddie says no...they don't like it, but thats what makes I believe a good parent, the strength to say no, even when its easier to say yes most of the time, the easiest option right now, is mostly not the easiest option in the long term.
My oldest is now 8, and still feel like Im saying no all the bloody time sometimes, but the rewards are really worth it, as most of the time, hes inquisitive, polite, creative and very tallented, knows and understands rules, boundries, and good behaviour is always rewarded

Hope that helped a little
Nicky x