As a photographer your likely going to hear that question a lot. When I got back into photography in 2007 that question was so easy...I shot cars and that was about it. Through the years and as I explored deeper and deeper into photography it stopped being so clearly defined. Just as my perspectives on life changed in these years so did my photography.
In a recent photo club meet I discovered many veteran photographers hit this realization, that while they have great passion for one subject within photography, as you grow you also grow more diversified; This is likely because there is this strong desire to learn and grow as an artist . I discovered when photographing landscape and portraits at first did not seem to influence my automotive photography, it did. Many of the critical items of photography such as composition, harmonious color, lighting and mood are learned at a greater scale when I diversified by subjects.
I look at my early automotive photos vs newer ones and I see a major style influence from my other photographic ventures.
Perhaps this is nothing more than refining ones artistic skill; like an athlete who trains and trains becoming more librated as their body becomes stronger...in this case building the artistic skill to push the creative soul into realms once thought beyond reach; mentally more agile than before.
I find myself intrigued a by a variety of subjects now and all the influence and passion I learned from another aspect of photography sits there in my bag of tricks for the next project.
If you are hitting a creative rut, talk to a fellow photographer about what types of projects they are shooting now or assist them. The more dynamic your become artistically the harder it will be for you to drop into a rut creatively.