The fact that science fiction and fantasy are collectively labelled speculative fiction lies at the root of this question. I've always been fascinated by the concept of "What if...?"
It started with Arthur C Clarke and being taken to see 2001: A Space Odyssey as a kid. I didn't understand it (does anyone still?) but that didn't matter, it got me wondering and trying to find answers to this Ultimate Question. I read everything Clarke wrote after that and got fascinated by the idea of what the future might hold. (So much of Clarke's prediction have come true too!)
As I grew older, my speculative interests morphed into novels that went beyond conventional sci-fi. Alan Dean Foster was a big influence, weaving worlds that lay in the hinterland of fantasy and sci-fi. I read H Rider Haggard which introduced me to mythology and legend in fantasy.
At the same time, as a teenage boy of the 60s, I got hooked by American comics like Astounding Stories which birthed the likes of DC and Marvel as well as TV programmes such as The Twilight Zone. They posed all kinds of possibilities that refused to be defined in conventional terms. But they went one step further, narrative was explored through the lens of the human condition. Whatever the concept, we were asked to look at how humans would react.
A preoccupation with superheroes followed. Fantastic Four, Spiderman, The Avengers and especially The X Men. The reason
was simple - what happens when people change? Become more than normal? X Men especially fascinated me - the idea of people mutating into something more than we are now, what they called Homo Superior, posed all kinds of questions for me. Questions that still exist. I've noticed my work tends to revolve around this issue now - my latest Work In Progress is based on this idea and is aimed at Young Adults. Probably the same types who are looking for answers to the questions I considered fifty years ago.
As an adult I've enjoyed a journey that's explored all facets of the Fantasy landscape but my favourite authors, I've realised, have had the same lasting impact on me: they've explored the human condition in a speculative context. They're not just stories with magic and swords and dragons and mystical quests. They might have involved those elements, but they've only done so as the story examines how people react to them.
Now, as an author myself, that need to find answers has inspired me to write stories which ask similar questions for my readers to answer. And so the circle continues.