When he was a young tyke, Martin could buy "dime store turtles" as easily as kids today can get their hands on penny candy or guns. So young Martin did what any of us would do: buy a s**tload of turtles before someone realized that giving out unlimited tiny turtles to kids is like making Godzilla honorary mayor of your cardboard town. He then set his turtles up in a toy castle, and since the '50s hadn't heard of reptilian ninjas yet, he pretended they were "knights, lords and kings." Before long, the little-but-still-bearded Martin began writing a "whole fantasy series about the turtle kingdom."
The thing about dime store turtles being nurtured by an absentminded kid is that most won't live to see retirement. Martin, utilizing some excellent coping mechanisms for a grade-schooler, incorporated the turtle deaths into his fantasy world, having them bump each other off in "sinister plots" in the war of succession for the turtle throne -- leaving the part where they'd wandered off and died underneath the refrigerator out of the history books.
Eventually, Martin grew out of his terrapin Hamlet, but he clung onto the valuable narrative lesson they'd taught him about the fragility of life. Which was the right thing to take away from that experience, because it wouldn't be the same show if Cersei Lannister was just munching on some lettuce for 45 minutes.