Question

Somebody told me once that whenever you ask God for something and he doesn't give you that, most probably that request is no good for you and you just don't know it yet. Someday you'll understand. Like a mother who won't give her child poison, no matter how hard the child cries for it. Someday, though not necessarily very soon, the child will understand.
Believe me I've been there many many times. Looking back in my life and wondering what the hell was I thinking? and of course thanking God for not giving me what I wanted.
But here is the thing that I don't understand. Isn't God the mighty of all? So he should be able to provide our request without any bad consequences. I mean after all, he is GOD!