That's something I learned in art school. I studied graphic design in Germany, and my professor emphasized the responsibility that designers and illustrators have towards the people they create things for.
We have eyes, and we're looking at stuff all the time, all day long. And I just think that whatever our eyes touch should be beautiful, tasteful, appealing, and important.
One day I think it's the greatest idea ever that I'm working on. The next day I think it's the worst that I've ever worked on - and I swing between that a lot. Some days I'm very happy with what I'm doing, and the next day I am desperate - it's not working out!
Let's put it this way: if you are a novelist, I think you start out with a 20 word idea, and you work at it and you wind up with a 200,000 word novel. We, picture-book people, or at least I, start out with 200,000 words and I reduce it to 20.
The hardest part is developing the idea, and that can take years.