There could be many reasons that an author writes. But a lot of it all comes down to one reason, there is a passion to write.
Inspirations come from all over. Being a fan of certain writers can help give you that particular want to write. People you know can, also, inspire and encourage you to follow your dreams in being an author.
Sometimes writing can be used as a means to cope with an extremely horrible situation or even to rid yourself of nightmares that are haunting you.
Maybe you want to be the next Stephen King, the next Michael Crichton, the next JK Rowling, who knows. If your inspiration is to be a great author, then I suggest you write what you want to write not what you think you should write.
Whatever your reason to write, have fun with your craft. Having fun writing your novels will ultimately show in the final product because you’ll put your best effort into it.