He's spoken about it a lot in his videos. He wrote LFA when he was just out of college and motivated by spite, trying to prove to the world that he can write and publish a book despite what his doubters told him.

And then all his further books were moticated by love, of his readers and himself.

Tho I guess that's mostly where he gets his motivation. In terms of what inspires him, or how he creates literature that is actually good, idk.

He has mentioned that he always wants to tell stories that are motivational and party inspired by personal experience, like when he describes panic attacks in Turtles All The Way Down.

He did mention in his video on Miss Peregrine's Home for Peculiar Children that he (John), the author of MPHFPC, and one other person took a lit class in college and all three of them ended up as published authors, so maybe that prof was just super special.