There's a few things at work for me.
Firstly, I always carry a camera, most often my little IID these days. So in some regards I am an "opportunist" photographer.
Secondly, I don't have huge amounts of time thanks to the day job to really focus on photography, so when I travel, a large part of the enjoyment comes from capturing what I see, and the novelty of it.
I rarely set myself a theme, as such, but themes do seem to surface in my work from time to time - I went through a period of doors/doorways, with people silhouetted therein. It wasn't until someone else pointed it out that I realised the (subconscious) theme, and then worked to it more consciously.
Other, lesser drivers include trying out new kit, or finishing a roll of film!
Regards,
Bill