The earliest science fiction book I can ever remember reading was "A Wrinkle In Time", long before I could even understand the themes the author was trying to express. The plot has been lost to me over time, but I do remember the wonder that came with exploring universes that suddenly became accessible to me, a kid who was basically limited to a couple blocks around his home.
I think it was in part this that inspired me to go back and reread the classics of science fiction over the last few years. Now I do appreciate some of the symbolism and deeper themes expressed by the authors, but ultimately its the sense of wonder at experiencing these other worlds that keeps drawing me back.