Every novel has a message. No matter what it is about, there is always something that the writer wants to say. That is what makes a story even more interesting to read, and as a writer, it makes the story even more interesting to write.
I highly recommend thinking about what you want to say before you grace the page with even a single word. Only because it saves time when editing. With the regular visits of new characters, saved time is a good thing.
The main thing you should ask yourself is: what do I want people to think about once they’ve put down my book? Then it’s just a matter of contemplating what events and characters you want to use to make people think about that. So the coming up with a message is actually the easy part. However, it is a lot easier now that you know what you’re working towards. And it’s also easier for your characters. Considering how we torture them already, it’s only the kind thing to do.