It would be easier for everyone if your "no" really meant "no" always and in all cases. But the thing is, shit, it’s not always so. And to recognize what and how is impossible.

Do you want to learn?
The simplest analogues from the life of men: Vase is offered vodka at work, he says sharp "no". Vase is offered vodka in the country, under a shale, Vasa says joyful "Yes". Although vodka and vasya are the same as the one offering. This "Not" means "Not here", "Not now". Change the conditions to more convenient.
Or here another. Vase is offered to pick up for 50p, he says "No". Offering 500r, Vasya says "Yes". "No" in this version means "your proposal is miserable to impossibility." This can relate to both the material sphere and the appearance and character of who is offered. Change the proposal to a more appropriate one.
Everything is quite simple, and it is not difficult to understand it if you want to.