There's just something that draws you in about the idea of creating your own world, with its own characters, backgrounds, storylines, beginnings and endings, and then sharing it with another person. I enjoy writing, period, I've been a fanfiction writer for years. But this, this is a little different. It's an interactive world. There's something more personal about that, I've enjoyed playing around in game worlds and story worlds for years, just as I've enjoyed creating them.
One reason I could cite is the characters. When I truly put my heart and soul into their creation, it feels like there's almost a little... life in them. Sharing them with others continues that life, and at times, even allows them to grow (did I mention that I roleplay?) and change and become more... real.
But even simpler than the characters... the feelings. Maybe I want to create the same level of emotion in others that these stories, these games, have brought out in me. Whether it's tears or laughter, the deep sigh of relief or smile of triumph when a likeable character -finally- reaches their goal, or the sympathetic pangs when a would-be hero falls flat on his face... that quiet, reflective, solemn pause you have when you realize you've just read something profound.... or just the simple sense of wonder at vicariously experiencing something new and unlike our own, real world.
I have enjoyed the human experience with these things, and I want to pass it on... after all, misery loves company, happiness celebrates it, but true joy demands it.