For me it was musical inspiration. When I was very young, I became aware of the concept of periodicity. I came to realize that most things in nature happened in regularly-recurring cycles; from molecular vibration to the orbits of entire galaxies. In other words, nearly everything in the universe has a frequency, and to me these many differing frequencies suggest notes, and the many differing notes being played simultaneously, chords. I was struck with the idea that all of creation is playing at a single song of unfathomable complexity and subtlety.
I am certain that no mortal being can ever know the whole song, but I am determined to learn as much of it as I can. When I was still quite young, I arrived at the idea of the "oscillating universe" as a cost logical model. Still later, I began hearing about "string theory". The more I learn, the more accurate that initial moment of childlike awe appears to be.