I have been asked this question a thousand times. My answer was always this: "Because I can escape from reality for a moment and create new worlds and creatures and situations for them to overcome."
But how did I get the inspiration? If you had asked me this two years ago, I would have said it just sprang from my always present love of unicorns. Now I know that this is wrong.
Has anyone ever seen the old movie (well, it's only 30 minutes, but I call it a movie) FIREFLY'S ADVENTURE? It a 'My Little Pony' thing for those who don't know. But it's got this magical land, a dark lord wanting to take over all, and all these other fantasy elements. My sister and I watched it SO much when we were little.
That's where my writing fantasy started. And, for those wondering, my love of science fiction came from the original series of Star Trek. Go figure. :)