Why don't friends want to pay for your services?

I am a Graphic Designer working for a small company. Though small we have big clients. I do freelance on the side. I get asked by friends often for logo's and vector art. The problem is they often want it for free or a really low price.

As a designer I can't spent so much time on a design for little pay. They don't understand how hard it is to make unique designs. After work, I am stressed, tired, mentally and creatively drained. For me to put that much time and effort into a design for free or a low price is unacceptable.